What other fish are in with them or could they possibly have fin rot? It is not a disease just for bettas but for all fish. The fish with less finnage will lose their fins much faster and the loss of a tail will kill any fish but if they are with fish that tend to be fin nippers and they had long or flowing fins they would be fair game to the nippers too. Since even one nip can get a case of fin rot going it would be perhaps even a combination of the two causes.
What other fish are in the tank?? some fish are fin nippers, also it may possibly be tail and fin rot, if so you may want to treat your tank for that. But before using any medications, make very sure that its not the first thing i mentioned.Sorry about your losses. *frown
Sometimes the nipping takes place at night but it could also be that it is just a very bad case of chronic fin rot. Tetras are really very bad at nipping though so you may want to try seperating the long finned fishes out from them since you don't have a complete school of them. Sometimes if they don't have a complete school they keep busy working on the other fish. It sometimes helps to keep a school of 6 or better 8 to keep them clustered.
Cannot promise it will stop the problem but it may.
It sounds like their fins are fine and you don't notice any nipped tails on the live fish right?
A lot of fish that die start to get their fins/eyes nipped after they die by other fish, algae eaters, shrimps, snails...etc if you don't find them and take them out right away. I am guessing your fish are fine, or at least that they did not die due to the fin nipping.
Yes that was my guess to after the fish die many snails and bottom feeders nip off the tails. But if many are dying at once it may be a non-visible sickness other than fin rot so I would recomend separating any fish with even the slightest signs of illness.
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