Corydoras Keep Dying, Most Times for Unknown Cause

Bluejay72 · Oct 20, 2022

I am new to the hobby of keeping freshwater fish. I bought a 20 gallon aquarium in February, cycled it and stocked it with live plants, and then got my first fish. I started with 5 emerald cories. I got a 6th cory that was supposed to be an emerald according to the local pet store, but turned out to be a brochis catfish (much longer snout and slightly different coloring than the emerald cories).

The brochis catfish died with fin rot, I’m guessing related to the betta I put in the aquarium who is a nipper. I removed the betta and put him in his own aquarium before the catfish died, but I didn’t notice the fin rot until it was too late.

I then had 2 cories die over the course of a few weeks. The only warning I had was that they turned more pale and didn’t participate in feeding the night before they died. No fin rot, no white spots, no damaged barbels, nothing.

After things were stable for 1-2 months, I got 6 new cories and put them into a quarantine tank. I ended up with 3 cories dying within 1-2 days. I unfortunately didn’t have enough hiding places initially and there was a dead cory in the tank at the pet store, in hindsight I should have avoided getting fish from that tank.

Of the 3 that survived, 2 are still alive. The albino died after a slower period of reduced color and reduced participation in feeding (it took 1 week). Another 2 of the original cories died. 1 was fast like the earlier cories I mentioned and the other was over the course of 2 weeks. Again, no physical changes or signs besides reduced color and reduced participation in feeding.

Currently, I have 3 cories that I would really like to keep alive. I am so sad and disappointed that I keep losing cories even though I have plenty of hiding places, plants, an air bubbler, good water parameters, and do weekly water changes.

PH is ~7
Ammonia is 0
Nitrite is 0
Nitrate is 0

I really want to have a healthy, happy aquarium and I need help on how to fix whatever is going wrong.

Byron · Oct 20, 2022

More than one member has had an experience like this, and obviously there is always a reason, so I hope I and other members can help you in finding it. First though, we need more data.

Do you know the GH (general hardness or total hardness) of your tap water? Yu may find this on the website of your water authority if you don't already know. It is not going to be the direct cause of the deaths (unless it is very hard), but it may be a contributing factor. Other readings given are good.

How often do you do partial water changes, and how much volume at each? What conditioner do you use (assuming you do)? Are any other substances or additives going in the tank water--plant nutrients, or anything at all?

The substrate looks like black sand, what kind (manufacturer) is it? Do you vacuum into the sand at the water changes?

What foods are you feeding? Be specific including the manufacturer for each.

What other fish, if any, are in this tank now? Removing the betta was very wise, this is not a community fish and should be on its own. It would have to be a fiarly aggressive betta to kill cories, but it can occur. So good move here.

Edit, something I forgot. The filter is an HOB, correct? And a bubbler. No issues heree, just checking. The wood makes me wonder, whre did it come from (meaning, fish store or collected yourself)? Hsve you seen any whitish fungus or mold patches on it?

It is after a time easy to spot increased respiration rates compared to normal, when the fish are not feeding or engaged in something...have you noticed this?

ella777 · Oct 20, 2022

Byron said:
More than one member has had an experience like this, and obviously there is always a reason, so I hope I and other members can help you in finding it. First though, we need more data.

Do you know the GH (general hardness or total hardness) of your tap water? Yu may find this on the website of your water authority if you don't already know. It is not going to be the direct cause of the deaths (unless it is very hard), but it may be a contributing factor. Other readings given are good.

How often do you do partial water changes, and how much volume at each? What conditioner do you use (assuming you do)? Are any other substances or additives going in the tank water--plant nutrients, or anything at all?

The substrate looks like black sand, what kind (manufacturer) is it? Do you vacuum into the sand at the water changes?

What foods are you feeding? Be specific including the manufacturer for each.

What other fish, if any, are in this tank now? Removing the betta was very wise, this is not a community fish and should be on its own. It would have to be a fiarly aggressive betta to kill cories, but it can occur. So good move here.

Random question, can plant fertilizers kill fish? I was wondering in case I ever add too much.

Byron · Oct 20, 2022

ella777 said:
Random question, can plant fertilizers kill fish? I was wondering in case I ever add too much.

Yes. This is why I always recommend using less rather than the level the manufacturer may "recommend." Some nutrients are heavy metals, and overdosing these (iron, copper, manganese, zinc) can be deadly, for plants actually as well as fish.

FranciscoB · Oct 20, 2022

Nítrate most likely is not zero, and if it is, that is suspicious. If the biological filter is working properly, it would result in nitrate accumulation. Perhaps a faulty test producing erroneous readings? How often and what volume do you change water? What plant fertilizer are you using and how much?

ella777 · Oct 20, 2022

Byron said:
Yes. This is why I always recommend using less rather than the level the manufacturer may "recommend." Some nutrients are heavy metals, and overdosing these (iron, copper, manganese, zinc) can be deadly, for plants actually as well as fish.

Okay thanks

Byron · Oct 20, 2022

FranciscoB said:
Nítrate most likely is not zero, and if it is, that is suspicious. If the biological filter is working properly, it would result in nitrate accumulation. Perhaps a faulty test producing erroneous readings? How often and what volume do you change water? What plant fertilizer are you using and how much?

May I correct the nitrate issue please? The case is quite different when live plants are involved. Plants need nitrogen, and most aquatic [plants (certainly those in the photo of the tank here) take up ammonia/ammonium as their preferred source of nitrogen. They also do this much faster than the nitrifying bacteria can, so if the plants are able to assimilate all of the ammonia the fish produce, plus that occurring from breakdown of the organics in the substrate, there is very little left for the nitrifying bacteria. You cannot measure this amount, it is so minimal, using our test kits. And since plants do not produce nitrite, and the nitrifying bacteria are not producing enough for us to test here too, there is no nitrate either, or very ,very little. Many with well planted low-tech or natural planted tanks see nitrates at zero for years. Provided one does not overload the system with too many fish, this can continue very well.

Ther are very few fish here, only 5 or 6 cories (unless the OP tells us different when my questions are answered) and even the few plants in this tank will easily deal with this.

FranciscoB · Oct 20, 2022

I overlooked the fact that there are indeed very few fish. However there are also not many plants. I guess we will find out what other answers might be about the tank and hopefully be able to help the OP.

AdoraBelle Dearheart · Oct 20, 2022

ella777 said:
Wait. Do plants remove nitrites/nitrates/ammonia? I'm confused, I bought lots of plants for my tank. They are all thriving, but the nitrates and nitrites have shot up. I have done lots of tests over the past few days. I have two brands of test strips, one of which is out of date (pretty sure they still work though).
Both tests were saying they went up, then one was showing 0 for nitrites and a little for nitrates (can't remember the exact number).
No fish have died and all seem very much alive. Are the tests wrong?
It seems the nitrites and nitrates have gone up since I put the plants in.
I've also got ammonia remover, would this help to remove nitrates and nitrites? I'm even more confused because its saying nitrites is rather high. I thought this would kill the fish straight away, it's been showing this for about four days.

@ella777 It might be better to make your own thread about this, as it risks veering the OP's thread off-topic.

FranciscoB said:
Nítrate most likely is not zero, and if it is, that is suspicious. If the biological filter is working properly, it would result in nitrate accumulation. Perhaps a faulty test producing erroneous readings? How often and what volume do you change water? What plant fertilizer are you using and how much?

Byron said:
May I correct the nitrate issue please? The case is quite different when live plants are involved. Plants need nitrogen, and most aquatic [plants (certainly those in the photo of the tank here) take up ammonia/ammonium as their preferred source of nitrogen. They also do this much faster than the nitrifying bacteria can, so if the plants are able to assimilate all of the ammonia the fish produce, plus that occurring from breakdown of the organics in the substrate, there is very little left for the nitrifying bacteria. You cannot measure this amount, it is so minimal, using our test kits. And since plants do not produce nitrite, and the nitrifying bacteria are not producing enough for us to test here too, there is no nitrate either, or very ,very little. Many with well planted low-tech or natural planted tanks see nitrates at zero for years. Provided one does not overload the system with too many fish, this can continue very well.

Ther are very few fish here, only 5 or 6 cories (unless the OP tells us different when my questions are answered) and even the few plants in this tank will easily deal with this.

FranciscoB said:
I overlooked the fact that there are indeed very few fish. However there are also not many plants. I guess we will find out what other answers might be about the tank and hopefully be able to help the OP.

@Byron You know I respect your knowledge a great deal, and that zero nitrAte tanks are possible - but I also find it highly unlikely to actually be the case here, even with only 5-6 fish in 20 gallons, given that all but one of the plants visible are slow growing java fern and an anubius. One small fast growing plant and a java fern isn't usually enough to lead to zero nitrAtes, especially given that most people are inclined to over feed fish, and there's also a high risk of a filter being cleaned in tap water for example, and causing mini-cycles. Worth investigating.

ETA: @Bluejay72 along with your water changing/substrate cleaning schedule, can you also describe how and how often you clean the filter and filter material, please? Do you switch out new cartridges for example? Or rinse anything under the tap?

Bluejay72 · Oct 21, 2022

Byron said:
More than one member has had an experience like this, and obviously there is always a reason, so I hope I and other members can help you in finding it. First though, we need more data.

Do you know the GH (general hardness or total hardness) of your tap water? Yu may find this on the website of your water authority if you don't already know. It is not going to be the direct cause of the deaths (unless it is very hard), but it may be a contributing factor. Other readings given are good.

How often do you do partial water changes, and how much volume at each? What conditioner do you use (assuming you do)? Are any other substances or additives going in the tank water--plant nutrients, or anything at all?

The substrate looks like black sand, what kind (manufacturer) is it? Do you vacuum into the sand at the water changes?

What foods are you feeding? Be specific including the manufacturer for each.

What other fish, if any, are in this tank now? Removing the betta was very wise, this is not a community fish and should be on its own. It would have to be a fiarly aggressive betta to kill cories, but it can occur. So good move here.

Edit, something I forgot. The filter is an HOB, correct? And a bubbler. No issues heree, just checking. The wood makes me wonder, whre did it come from (meaning, fish store or collected yourself)? Hsve you seen any whitish fungus or mold patches on it?

It is after a time easy to spot increased respiration rates compared to normal, when the fish are not feeding or engaged in something...have you noticed this?

GH of tap water based on the website: It typically has a total hardness of 3–8 parts per million (ppm), or ¼ to ½ a grain of hardness per gallon
Note: I am planning to buy the test so I can check for myself but it’s out of stock at the local stores so I have to order online.

Updated readings as of today (11 days since last water change of 20%)
Ammonia: 0
Nitrate: 0
Nitrite: 0
pH: 7.6 (I misremembered the pH, I check it pretty infrequently)
I test (all except pH) before I do water changes. Usually i do one every 7-8 days with 0 for all values. I was late this week because work was very busy this week.

Water conditioner (see images): Prime 2 drops per gallon and Stress Coat + 5 drops per gallon.

Shrimp essentials new additive as of 1 week ago so no correlation with the Cory deaths.

See picture of what I think the sand is, but I can’t remember exactly since it has been since February when I started my aquarium.

Wood: came from fish store, no growths. See updated photo (taken today) of my aquarium since I removed a few things since the photo I posted (taken 3-4 weeks ago)

Yes, I noted increased respiratory rates just before they died when the behavior and color changes happened

Bluejay72 · Oct 21, 2022

FranciscoB said:
Nítrate most likely is not zero, and if it is, that is suspicious. If the biological filter is working properly, it would result in nitrate accumulation. Perhaps a faulty test producing erroneous readings? How often and what volume do you change water? What plant fertilizer are you using and how much?

Every 7-8 days ~20% water changes, I vacuum the sand
API Freshwater Master test kit, I do have plants so maybe that’s why it’s all 0?
No plant fertilizer

Bluejay72 · Oct 21, 2022

Byron said:
More than one member has had an experience like this, and obviously there is always a reason, so I hope I and other members can help you in finding it. First though, we need more data.

Do you know the GH (general hardness or total hardness) of your tap water? Yu may find this on the website of your water authority if you don't already know. It is not going to be the direct cause of the deaths (unless it is very hard), but it may be a contributing factor. Other readings given are good.

How often do you do partial water changes, and how much volume at each? What conditioner do you use (assuming you do)? Are any other substances or additives going in the tank water--plant nutrients, or anything at all?

The substrate looks like black sand, what kind (manufacturer) is it? Do you vacuum into the sand at the water changes?

What foods are you feeding? Be specific including the manufacturer for each.

What other fish, if any, are in this tank now? Removing the betta was very wise, this is not a community fish and should be on its own. It would have to be a fiarly aggressive betta to kill cories, but it can occur. So good move here.

Edit, something I forgot. The filter is an HOB, correct? And a bubbler. No issues heree, just checking. The wood makes me wonder, whre did it come from (meaning, fish store or collected yourself)? Hsve you seen any whitish fungus or mold patches on it?

It is after a time easy to spot increased respiration rates compared to normal, when the fish are not feeding or engaged in something...have you noticed this?

Forgot to add, no other fish. Just 5 shrimp (added last week) and 1 snail

Byron · Oct 21, 2022

This thread has become significantly hijacked, so going forward I will only be responding to the OP's questions/issues. Otherwise it will get hopelessly tangled and be of little value to him/her.

GH, this is good, no issues there.

Ammonia, nitrite, nitrate at zero are excellent. No issues here.

Water conditioner...Do not use StressCoat, it has aloe vera known to affect fish gills. You only need one conditioner anyway, one that detoxifies chlorine/chloramine (usually) and heavy metals. Personally I would not use Prime, it messes around with other things best left alone. The API Tap Water Conditioner is probably the best we have. I am not saying this is causing cory deaths, but I am saying this conditioner is not as good as many think, and the TWC is preferable and safer (I could go into details over what is in Prime but it would hijack the issue more, so not needed).

Shrimp Essentials...not sure what is in this but I assume it is some sort of mineralization for shrimp. I would not use this in a tank with cories. There are soft water shrimp you could get. Cories will consider shrimp to be food (it is their second most natural food in the habitats, after insects/insect larvae). Having said this, I do agree this is not likely the issue, but once again it is best avoided.

Increase the volume of each water change to at least 50-60% of the tank volume (at one change, not spread out). Do a good clean into the open areas of the substrate. Sometimes clean water does wonders.

Everything is looking pretty good, but I am still questioning the wood...those lightish patches I can see, you are certain they are not a bit slimy? I can only go from photos and often it is not the same as seeing it "live" as it were.

You did not mention the foods you are feeding them. This is important.

It is always possible the fish have something from the store, you did say there was a dead fish in the tank. I would check to see if the cories left in the store are having issues like your fish.

Slaphppy7 · Oct 21, 2022

Stress Coat contains aloe vera = not good for fish

Bluejay72 · Oct 21, 2022

Byron said:
May I correct the nitrate issue please? The case is quite different when live plants are involved. Plants need nitrogen, and most aquatic [plants (certainly those in the photo of the tank here) take up ammonia/ammonium as their preferred source of nitrogen. They also do this much faster than the nitrifying bacteria can, so if the plants are able to assimilate all of the ammonia the fish produce, plus that occurring from breakdown of the organics in the substrate, there is very little left for the nitrifying bacteria. You cannot measure this amount, it is so minimal, using our test kits. And since plants do not produce nitrite, and the nitrifying bacteria are not producing enough for us to test here too, there is no nitrate either, or very ,very little. Many with well planted low-tech or natural planted tanks see nitrates at zero for years. Provided one does not overload the system with too many fish, this can continue very well.

Ther are very few fish here, only 5 or 6 cories (unless the OP tells us different when my questions are answered) and even the few plants in this tank will easily deal with this.

Byron said:
This thread has become significantly hijacked, so going forward I will only be responding to the OP's questions/issues. Otherwise it will get hopelessly tangled and be of little value to him/her.

GH, this is good, no issues there.

Ammonia, nitrite, nitrate at zero are excellent. No issues here.

Water conditioner...Do not use StressCoat, it has aloe vera known to affect fish gills. You only need one conditioner anyway, one that detoxifies chlorine/chloramine (usually) and heavy metals. Personally I would not use Prime, it messes around with other things best left alone. The API Tap Water Conditioner is probably the best we have. I am not saying this is causing cory deaths, but I am saying this conditioner is not as good as many think, and the TWC is preferable and safer (I could go into details over what is in Prime but it would hijack the issue more, so not needed).

Shrimp Essentials...not sure what is in this but I assume it is some sort of mineralization for shrimp. I would not use this in a tank with cories. There are soft water shrimp you could get. Cories will consider shrimp to be food (it is their second most natural food in the habitats, after insects/insect larvae). Having said this, I do agree this is not likely the issue, but once again it is best avoided.

Increase the volume of each water change to at least 50-60% of the tank volume (at one change, not spread out). Do a good clean into the open areas of the substrate. Sometimes clean water does wonders.

Everything is looking pretty good, but I am still questioning the wood...those lightish patches I can see, you are certain they are not a bit slimy? I can only go from photos and often it is not the same as seeing it "live" as it were.

You did not mention the foods you are feeding them. This is important.

It is always possible the fish have something from the store, you did say there was a dead fish in the tank. I would check to see if the cories left in the store are having issues like your fish.

Going forward, whenever I get new fish (I won’t get any until things are totally sorted out), I will avoid tanks with dead fish and if I have any fish that die within 1-2 weeks I will follow up with the fish stores.

Foods are shrimp pellets and sinking wafers pictures here. I used to do 2 wafers or 3 pellets. I was told by the fish store I was over feeding so I dropped it to 1 wafer or 1-2 pellets every other day. Then the internet indicated I should be doing 1 wafer or 1 pellet 6 days/week. This was when I had 4-6 cories. Now I’m doing 1 wafer or 1 pellet every other day since I have 1 large and 2 small cories.

Good to know about stress coat!

Something I can’t believe I forgot to mention is that I have what I believe to be blue/green algae (Cyanobacteria). I didn’t realize this wasn’t algae at first so I used API algae fix for 1-2 weeks and I think this killed the 2nd and 3rd Cory as well as my 4 nerite snails (white patches on the wood are nerite snail eggs I believe). I was so upset about the first Cory dying that I was trying to treat every possible issue. I likely caused more issues by doing this. I did use a carbon filter and 50% water changes 2 times/week for a couple of weeks then moved to 25% water changes and removed the carbon to clear the algae fix as recommended by the fish store.

So to recap. First Cory died with fin rot probably from betta nipping him. I removed the betta, used algae fix, 2 cories and 4 nerite snails, I used carbon and bigger water changes to remove the algae fix, then I had 3 more cories die of unknown reasons (doesn’t include the 3 that died in the quarantine tank). So 3 of the deaths are not really easily justifiable as I look back.

Let me know if I missed something!

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Corydoras Keep Dying, Most Times for Unknown Cause

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