Unknown Fish Death

In a tank that size with the number of fish you already had, adding 2 new platies and 2 shrimp should have not made any impact at all on the water conditions. There would have been only a tiny % increase in the bioload so the filter would have coped easily.
Did you do anything else to the tank - new decor, new plants? Did anyone use some sort of spray near the tank, and it's co-incidence that the new fish were bought at the same time? Or use an air freshener, do any painting?

If not, that just leaves some type of disease as the cause of the fish deaths, though what disease it could be to strike so quickly, I don't know. The fact that the dead fish look fine suggests an internal infection. The problem is that there are few meds available in the UK to treat internal bacterial infections without a vet's prescription. And I don't think there is anything that treats viral infections, with or without a prescription.

I would do a much larger water change than you have been doing, more like 50% minimum (not an easy task with that tank size). You could try using an anti-internal bacteria infection - the way things have been going it shouldn't do any extra harm. Interpet make such a med, and as eSHa 2000 claims to treat dropsy, that might also work.

I'm sorry I can't be more help, it must be very upsetting watching all those fish die.

I would give serious thought to getting a quarantine tank, and keep all new fish (and shrimps) in that for at least two weeks, preferably four.

I didn't pay much attention to the death of the shrimp simply because of problems I had last year. I bought batches of cherry shrimps from three different shops and every single one of them died, some straight away some after a couple of weeks. I know from personal experience that shrimps dying is not unknown.


I have read elsewhere that the effects of these pockets of gas are a fishkeeping myth, they don't actually kill fish. Is there any definite proof (as opposed to hearsay) one way or the other?

I also suspect a chemical reason which is why I asked about sprays, air fresheners etc, and any new decor which could have been contaminated.
Because I don't have plants growing in the substrate I have fairly shallow sand in all my tanks so I have no experience of nasty things that grow in deep substrate.

We need D1976p to come back and tell us of anything he's done to the tank or in the same room as the tank.

Shrimps are expert at hiding so if you haven't found any bodies they may still be alive - though as you say, they could have been eaten. And I would include the mollies and platies among the shrimp eating suspects.
They are also good at escaping, another possibility. Shrimps will climb out of a tank if there is something not to their liking.
Something about shrimps to bear in mind - they shed their skins as they grow and leave empty shells behind. They look like hollow shrimps. If yours are still alive, don't worry if you find these shells, they aren't dead shrimps. Shrimps usually turn pink when they die.

If you clean the gravel every week (do you clean right to the bottom of it?) that lessens the chances of it being something nasty escaping from there, but Richard may have suggestions of things that can accumulate even with cleaned gravel.

A nice big water change will help. If there is something in the tank that will dilute it, lowering the likelihood of it killing more fish.