Losing corys...

[vprice1030]

Hi all, I'll start with some background...
I have a 40 gallon breeder with 10 cardinal tetras, 10 rummynose tetras, 5 zebra danios, a bushynose pleco, and 6 fairy corys. I feed a combination of flake food, sinking shrimp pellets, and the occasional bloodworms and algae wafers.

Recently, I've been starting to lose corys- I lost two within a couple of days, and a third (out of an original school of 6) is behaving lethargically and appears to have some degree of fin rot. I've had them for about three weeks and all seemed to be going well for the first two weeks. I got them from an extremely reputable LFS and drip acclimated them.

My WQ parameters are as follows:
pH: 6.8
Ammonia: 0
Nitrates: <5
Nitrites: 0
Temp: 77
Hardness: 2

The tank has been established for about four months (and was started with filter media, substrate, etc from a smaller tank that had been established for several years). I've been testing water parameters 2x/weekly during this 4 month period and the only thing that has changed is a slight rise in nitrates from 0 to something less than 5 (it's between the 0 and 5 on the API liquid test kid key). I do 25% water changes weekly, with a thorough sand/gravel vac.

Is this fin rot that's killing my guys? Is a slight rise in nitrates enough to cause this much stress? And lastly...is there anything I can do, beyond aggressive water changes?

 

[Byron]

I don't comment much on disease issues as I have so little experience and guessing can make things worse, so my comments are more general. Degenerating fins can be due to water issues, nipping, bacterial, fungus...etc. It is always possible they brought something with them.

I assume the species is Corydoras atropersonatus. These will thus be wild caught unless you get them from a local breeder. There is nothing wrong with your water parameters, and nitrates no higher than 5 ppm will not cause sudden death like you describe. Nitrogen values seem OK.

What is the substrate material? You mention "sand/gravel" cleaning, but is it sand or gravel or both? And what type, specifically; is it one of the plant substrates?

Do they have places to hide, such as under chunks of wood? Issue here is avoiding additional stress, and fish out in the open can be quite stressed.

You should do larger weekly water changes, 50-60% of the tank volume once each week. At present, I would certainly recommend a major water change now, and perhaps another tomorrow or the day following. Nothing benefits like a water change.

 

[vprice1030]

Hi Byron, thanks for your response! I have aquarium sand covering 75% of the bottom, and 25% fine polished pepple gravel that I transferred from the old tank.

There are several hollow logs, rock caves, and a couple of terra cotta pots, as well as fairly heavy planting (unfortunately at this point, they're artificial plants). The healthy corys don't really seem to use them often, but the one who is not doing well spends much of his time in one of the logs.

I will definitely up the volume of the water changes, as well as do more aggressive ones (I've done three 25% changes in the last few days) today and tomorrow. Thank you!

No tankmates seem to be affected, and I have not observed any fin nipping.

 

[Colin_T]

Do a 75% water change and gravel clean the sand each day for a week.
Clean the filter if that hasn't been done in the last 2-3 weeks.
Post a picture if the sick fish.

 

[vprice1030]

K, he's hanging in there! He's obviously stressed, but still seems to be attempting to eat and move around a little. The other three still seem very healthy. I did a 60% water change yesterday and will do another today. Fingers crossed. The filter cartridge was replaced last week.

 

[Jack TH]

Hmm. This might not have anything to do with what you've done. If they've been wild-caught then they've had to have been transported at some point, where they could have been met by stressful conditions. Sometimes fish will look fine for a couple of weeks, then apparently catch something and become ill, when really they were ill the whole time and just didn't show symptoms until a couple weeks after infection, in which case it's not your fault. Pics are definitely needed, as lethargy and fin rot are symptoms of a systemic disease.

The filter cartridge was replaced last week.

I hope you kept some filter media from the previous filter cartridge, otherwise that's what's led to these problems. Generally filter cartridges don't need to be replaced until the sponge is falling apart, and even then you should keep some of it so that the cycle is still there. They just tell you to replace every [insert time frame here] so you keep buying their product. There's no regulation in this industry. If you actually do that, you will kill your cycle every time and these kinds of problems will happen to no end.

Before people ask, the reason why you aren't getting any nitrite or ammonia levels if you didn't keep any old filter media is because you've been doing big water changes, and you already have a lot of fish which will facilitate the growth of a new cycle. Or it could be that your water parameters are inaccurate, as 5ppm is a very low number only achieved with a ton of plants and very few fish. Or you tested just after changing the water, which gave you accurate but deceptive results. Please confirm this OP.

 

[vprice1030]

Hmm. This might not have anything to do with what you've done. If they've been wild-caught then they've had to have been transported at some point, where they could have been met by stressful conditions. Sometimes fish will look fine for a couple of weeks, then apparently catch something and become ill, when really they were ill the whole time and just didn't show symptoms until a couple weeks after infection, in which case it's not your fault. Pics are definitely needed, as lethargy and fin rot are symptoms of a systemic disease.


I hope you kept some filter media from the previous filter cartridge, otherwise that's what's led to these problems. Generally filter cartridges don't need to be replaced until the sponge is falling apart, and even then you should keep some of it so that the cycle is still there. They just tell you to replace every [insert time frame here] so you keep buying their product. There's no regulation in this industry. If you actually do that, you will kill your cycle every time and these kinds of problems will happen to no end.

Before people ask, the reason why you aren't getting any nitrite or ammonia levels if you didn't keep any old filter media is because you've been doing big water changes, and you already have a lot of fish which will facilitate the growth of a new cycle. Or it could be that your water parameters are inaccurate, as 5ppm is a very low number only achieved with a ton of plants and very few fish. Or you tested just after changing the water, which gave you accurate but deceptive results. Please confirm this OP.

I should clarify, that I did not replace the sponge- just the carbon. I am now very curious what the "correct" thing to do with HOB filters is- do I need carbon? No?

I don't test the water immediately after changes but most often in the morning (and then I do the changes in the evening when I get home from work). All of the other fish seem very healthy.

I'll wait and see if the others show symptoms- unfortunately my third guy passed away. The other three seem to be fine, but I'll get pictures ASAP if not.

 

[Jack TH]

I should clarify, that I did not replace the sponge- just the carbon. I am now very curious what the "correct" thing to do with HOB filters is- do I need carbon? No?

To be honest, the carbon is really unnecessary. It's only real use is to get rid of medications. You don't need it in your filters, and it can be a hindrance because if it's part of the sponge then you kill some of your cycle if you need to remove it for any reason. You can safely get rid of the carbon and if it is part of the sponge, replace it with regular sponge. If it was part of the sponge it would have damaged but not killed your cycle, which wouldn't have helped matters.

I don't test the water immediately after changes but most often in the morning (and then I do the changes in the evening when I get home from work). All of the other fish seem very healthy.

Ok, thanks for clarifying.

I'll wait and see if the others show symptoms- unfortunately my third guy passed away. The other three seem to be fine, but I'll get pictures ASAP if not.

RIP to that guy. If it doesn't happen to any others then there's nothing you can really do about it. Sometimes things like this happen to fish if they get stressed, some fish are just funny like that. I suspect something in the water chemistry changed, which caused the cories to get stressed, and then the fin rot attacked them, which gives them an appearance of an apparently systemic infection when really they are just badly stressed and have attracted some fungi/bacteria. Again, nothing you can really do.

Do you by any chance live in the UK? The tetras here are super strong and healthy, I had a whole batch of glowlights completely recover from pretty bad fin rot with no treatment whatsoever. Since they are your only other fish and if you did have some of this amazingly strong tetra stock, I would imagine the tetras wouldn't get affected by something like this if it was a complication of stress.