New fish dying within hours/days

Tank size: 20 gallons
pH: 7.6
Ammonia/nitrite/nitrate: 0.25 / 0 / 5ppm (new fish, so the bacteria haven't caught up yet I guess, fully cycled, will do a water change to get it back down)
GH: 5 degrees
Temperature: 79 degrees F
Moderately planted
Lights on for ~6 hours per day
No medications or chemicals are used in the tank besides prime water conditioner
Tank has a lid, so the fish aren't jumping out.

I bought 8 rice fish 3 days ago, they were vibrant, active, and looked healthy at the store. They had been at the store for about 2 weeks before I got them, so they were quite settled in there. I acclimated them to my tank conditions by pouring them into a bucket, then adding a small scoop of water from my tank and putting it into their bucket every 5 minutes or so. Once the bucket was half full, I poured out most of the water and filled it up one scoop at a time until it was half full again, then put them into the tank. I left the lights off so they could adjust.

At first they were all active and swimming about, quite bright still despite the move. After about 2 hours, I noticed one was missing. I found it dead at the side of the tank, so I scooped it out. I thought it was normal, since it's not unusual for a fish or two to not make it through the acclimation, but then I woke up the next day to 2 more dead. Then more kept dying, and now I only have 3 left out of the original 8.

I also have 3 panda garras and 4 otocinclus that I got a few weeks ago that are all doing fine, so I'm not really sure why my new fish are dying. I also tried to get 8 CPDs 2 months ago, but they gradually disappeared until there were none left after 6 weeks (which is why I decided to try rice fish this time).

I did a fishless cycle before adding any fish according to the guidelines on here (took a few months), and the tank has been running for over a year now, so it's quite established by now. I'm not really sure why I can't keep any new fish alive in here : \ Any help would be greatly appreciated!

Included 2 pictures of some of the rice fish (not sure if those are the ones that died or survived, they're from the other day), and a picture of the whole tank they're in.

What is your water change routine - frequency and amount? Very unlikely to be old tank syndrome but need to check.

How good is the store you're buying fish from? I know they look healthy at first but it's quite possible that you have had a couple of bad batches. Are they a chain store or specialists? They say they quarantine/settle the fish at the store but have you checked into this? Are there always fish labelled 'not for sale', do you see the same fish over the weeks? Do you see dead or sick fish? Do they routinely keep antibiotics in the tanks?

Lastly, I like to use the 'plop and drop' method to transfer fish from the bag. Have a largeish fish net balanced on a bucket. Cut the bag open and tip the contents into the net. Place the fish in your tank and discard the water from the bucket. The logic is that the sooner the fish are in fresh, clean water, the better it is. They cannot acclimatise by gradually changing over the water for a few minutes/hours anyway; the bag water potentially has high levels of nitrite and ammonia, as well as pathogens; and the temperature will be unstable in a bucket. I agree that leaving the tank lights off for a while is beneficial.

Naughts said:

What is your water change routine - frequency and amount? Very unlikely to be old tank syndrome but need to check.

How good is the store you're buying fish from? I know they look healthy at first but it's quite possible that you have had a couple of bad batches. Are they a chain store or specialists? They say they quarantine/settle the fish at the store but have you checked into this? Are there always fish labelled 'not for sale', do you see the same fish over the weeks? Do you see dead or sick fish? Do they routinely keep antibiotics in the tanks?

Lastly, I like to use the 'plop and drop' method to transfer fish from the bag. Have a largeish fish net balanced on a bucket. Cut the bag open and tip the contents into the net. Place the fish in your tank and discard the water from the bucket. The logic is that the sooner the fish are in fresh, clean water, the better it is. They cannot acclimatise by gradually changing over the water for a few minutes/hours anyway; the bag water potentially has high levels of nitrite and ammonia, as well as pathogens; and the temperature will be unstable in a bucket. I agree that leaving the tank lights off for a while is beneficial.

Click to expand...

I change 25% of the water every week, and make sure the new water is the same temperature as the tank and dechlorinated with prime.

The store is a small aquatic specialty store. They do have the same fish there for weeks/months for a few of the bigger fish like the bichir and some really nice big chiclids and clownfish. I sometimes see maybe one dead molly or guppy, but never a lot of dead or sick fish. I'm not sure about any antibiotics or anything they use there, though.

I'll try your method next time I get some and see how it goes! Thank you for the reply!

For small fishes and invertebrates.

I move them from the bag with the water to a 1/2 gallon fish bowl on a heating pad and air pump with only air line and a weight at the end (no air stones) and drip acclimate up to 4 times the content of the bag.

Then drop everything trough a net over a sink and plop in tank.

If your water is not completely different or unsuitable, You should not lose anything. few hours / days after introduction of apparently healthy specimens.

The fish look fine and don't appear to have a disease.

If fish die shortly after they are added to the aquarium, it's either poisoning from something in the tank or too much change in water chemistry (eg: the fish came from water with a pH of 6.0 and went into water with a pH of 8.0, or the GH/ KH was completely different).

If the other fishes that have been in the tank for a while are ok, it could still be poisoning whereby those fish have adapted to the poison but are on the edge of dying.

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Do you gravel clean the substrate when you do a water change?

What sort of filter is on the tank?
How often and how do you clean the filter?
Do you have carbon in the filter?
If yes, how often do you change the carbon?

Do you have buckets specifically for the fish tank?
Has anyone used the fish buckets for anything else?

Do you use hand sanitiser or have any moisturiser cream, oil, perfume, grease or anything else on your hands that could wash off into the water?

Did you do a water change on your tank a few days before adding the fish?

Do you have chlorine or chloramine in your water supply?
You can find this information on your water company's website or by phoning them.

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If you don't have carbon in the filter, perhaps add some and replace it every month. Try to use Activated Carbon or Highly Activated Carbon because they work better and last longer than plain Carbon.
If there is a poison in the water, carbon should remove it and stop the fish dying.

Make sure your hands and arms are free of any chemicals or hand sanitiser.

Make sure any buckets or hoses used on the tank are only used on the tank. I recommend having a couple of new white buckets and using a permanent marker to write "FISH ONLY" on them and not let anyone use them for anything else.

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You need a picture on the back of the aquarium to help the fish feel more secure.

You could increase the lighting time to 10-12 hours a day. The Ambulia has small leaves and this is normally from lack of light.

TURNING LIGHTS ON AND OFF
Stress from tank lights coming on when the room is dark can be an issue. Fish don't have eyelids and don't tolerate going from complete dark to bright light (or vice versa) instantly.

In the morning open the curtains or turn the room light on at least 30 minutes (or more) before turning the tank light on. This will reduce the stress on the fish and they won't go from a dark tank to a bright tank instantly.

At night turn the room light on and then turn the tank light off. Wait at least 30 minutes (or more) before turning the room light out. This allows the fish to settle down for the night instead of going from a brightly lit tank to complete darkness instantly.

Try to have the lights on at the same time each day. Use a timer if possible.

If the light unit is programmable, have it on a low setting for the first 30-60 minutes and increase the brightness over time. Do the opposite in the evening and gradually reduce the light for the last 30-60 minutes before lights out.

If you don't have live plants in the tank, you only need the light on for a few hours in the evening. You might turn them on at 4 or 5pm and off at 9pm.

If you do have live plants in the tank, you can have the lights on for 8-16 hours a day but the fish and plants need 8 hours of darkness to rest. Most people with live plants in their aquarium will have the lights on for 8-12 hours a day.

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LIGHTING TIMES
Most aquarium plants like a bit of light and if you only have the light on for a couple of hours a day, they struggle. If the light doesn't have a high enough wattage they also struggle. Try having the tank lights on for 10-12 hours a day.

If you get lots of green algae then reduce the light by an hour a day and monitor the algae over the next 2 weeks.
If you don't get any green algae on the glass then increase the lighting period by an hour and monitor it for a couple of weeks.
If you get a small amount of green algae then the lighting time is about right.

Some plants will close their leaves up when they have had sufficient light. Ambulia, Hygrophilas and a few others close their top set of leaves first, then the next set and so on down the stem. When you see this happening, wait an hour after the first few sets of leaves have closed up against the stem and then turn lights off.

Plant lights should have equal amount of red and blue light and a bit less green light.

Thank you for the in-depth reply!

I have an HOB filter, which I never clean except for maybe scraping off some algae that grows off the spout in a gross mess. I don't have any carbon, so I'll look into getting some!

My buckets were bought exclusively for the tank and nobody else uses them, I'm quite sure of that. I don't use any hand creams or anything, and I make sure to rinse my hands very thoroughly with plain water before putting them in the tank.

I didn't do a special water change before adding the fish, just my normal weekly change a few days beforehand. I usually vacuum the gravel every other water change or so.

My tap water has chloramine, so I dose extra prime when I change the water.

I'd like to have the lights on for longer because I know my plants are struggling, but if I have them on for even one extra hour I get a TON of black beard algae all over every plant and all the driftwood

Thank you again!

Established biological filters should be cleaned at least once a month. Wash the filter media/ material in a bucket of aquarium water and re-use the media. Tip the bucket of dirty water on the lawn.
The filter case and motor/ impellor can be washed under tap water.

Everything else seems fine so clean the filter, add some carbon, wait a month or so and see how things are going. Then maybe try some more fish.

If you have a quarantine tank, it can help when getting new fish because there is less in the tank to possibly cause problems.

Any reason why you are running your tank that higher temperature? My CPDs are in tanks from 70f-74
All the fish you have listed will be comfortable at 74f

Thank you very much Collin_T for all your help! I'll do that!

No, there isn't really a reason for the higher temperature. I'll turn it down, thanks!

Any idea if the Medakas (rice fish) were in a cold water (unheated) aquarium at the pet shop?
Maybe call the shop and find out. If they were and you put them in a tropical tank the temperature difference might be a contributing factor. However, there's still the issue with other fishes dying.