All fish rapidly died in new aquarium - water parameters fine?

[Fishywishy333]

Hi all,

I have recently fishless cycled a new 90 litre and after a couple of stable readings (0ppm ammonia and nitrite, 10ppm nitrate) decided to add 6 black neons yesterday.

I acclimatised them as normal, adding tank water to their bag gradually etc. All seemed fine for the first 30 mins or so, then I noticed the fish were hanging around the surface. As I know this can be an indication of poor water quality, I retested and all parameters were the same. My other thought was lack of aeration, although my filter is splashing and creating bubbles so I'm not sure this was the problem. At this point I'd also left the lid open to allow gas exchange.

After a couple of hours, all fish rapidly deteriorated, showing erratic swimming behaviours, then died.

I'm gutted as I've set up many aquariums over the years and never experienced something like this.

Any ideas? My only other theory is that my pH is a little high (7.8) and this may have increased NH3 ammonia if there was trace amounts? Plenty of water conditioner was used, so I reckon this rules out heavy metals.

 

[GaryE]

The number one cause of death in new fish is conditions before you received them - either shipping stress or diseases in the crowded stores. Even the best stores can have problems, as they depend on wholesalers to bring the fish to them. The worst stores, the large national chains in North America, often have very damaged fish. You said "gutted" so I'll take a guess at the UK - you seem to have some better chains, but some are as bad as the US ones.

If the fish were scorched by ammonia before you got them, all the test kits in the world will tell you nothing. Rather sensibly if not ethically, most retailers will let you take the blame, but you're probably innocent. It happens.

 

[MattW]

Sorry for the loss. I'm interested in where you got the fish as well. Do you have any plants or driftwood in the tank as well? There are some plant sellers in the UK who dip their plants in chemicals as a precaution for pests/parasites.

 

[Fishywishy333]

The number one cause of death in new fish is conditions before you received them - either shipping stress or diseases in the crowded stores. Even the best stores can have problems, as they depend on wholesalers to bring the fish to them. The worst stores, the large national chains in North America, often have very damaged fish. You said "gutted" so I'll take a guess at the UK - you seem to have some better chains, but some are as bad as the US ones.

If the fish were scorched by ammonia before you got them, all the test kits in the world will tell you nothing. Rather sensibly if not ethically, most retailers will let you take the blame, but you're probably innocent. It happens.

Sorry for the loss. I'm interested in where you got the fish as well. Do you have any plants or driftwood in the tank as well? There are some plant sellers in the UK who dip their plants in chemicals as a precaution for pests/parasites.

Thanks both for your replies. I bought the fish from Maidenhead Aquatics (not many options around me unfortunately) and plants from ProShrimp. I would be tempted to think the fish weren't in the greatest health being from a chain, but they were actually the last 6 from a batch which arrived in Jan 2025, so had done pretty well until now.

Matt, that's an interesting suggestion about the plants, I can't remember if I did rinse them in this case, so it is possible. Although I just googled it and looks like ProShrimp dont use pesticides.

 

[Essjay]

ProShrimp does sell shrimp safe plants so they won't be treated with anything that would harm fish. It's plants grown in the far east which tend to be treated; those grown by European sellers are not treated.


The issue arises because the EU banned all species of Pomacea snail and insisted that plants from areas where these snails are endemic be treated with snail killing chemicals. The UK may have left the EU but far east growers will probably send the same plants here as to the EU.

 

[MattW]

Thanks both for your replies. I bought the fish from Maidenhead Aquatics (not many options around me unfortunately) and plants from ProShrimp. I would be tempted to think the fish weren't in the greatest health being from a chain, but they were actually the last 6 from a batch which arrived in Jan 2025, so had done pretty well until now.

Matt, that's an interesting suggestion about the plants, I can't remember if I did rinse them in this case, so it is possible. Although I just googled it and looks like ProShrimp dont use pesticides.

As @Essjay said proshrimp plants are safe. I can back that up as I have spent many hundreds of £s on that site and have not had any issues. With the fish being in that shop since Jan last year, they must have gone through many medications. My local maidenheads follow up with treating the tank systems after every delivery or two. I would assume and hope this is also done company-wide. I wonder if there is a hidden chemical in your tanks water?

 

[Fishywishy333]

ProShrimp does sell shrimp safe plants so they won't be treated with anything that would harm fish. It's plants grown in the far east which tend to be treated; those grown by European sellers are not treated.


The issue arises because the EU banned all species of Pomacea snail and insisted that plants from areas where these snails are endemic be treated with snail killing chemicals. The UK may have left the EU but far east growers will probably send the same plants here as to the EU.

As @Essjay said proshrimp plants are safe. I can back that up as I have spent many hundreds of £s on that site and have not had any issues. With the fish being in that shop since Jan last year, they must have gone through many medications. My local maidenheads follow up with treating the tank systems after every delivery or two. I would assume and hope this is also done company-wide. I wonder if there is a hidden chemical in your tanks water?

Thanks both. In this case I reckon I will do a full water change and keep retesting my parameters for a while before I get any new fish.

 

[gwand]

I once had a pair of a. boreli die in a well established quarantine tank during a six week quarantine. Cause unknown. The pair were bought from The Wet Spot, a highly esteemed on line seller. Sometimes there are magical forces at work.