If the issue was a disease, most of them are species related, meaning different fish are susceptible to different germs that normally live in a tank even with very healthy fish. In a community tank it is very often that just one species of fish is affected.
However, if it were me, when getting the new fish, I'd give everyone a flubendazole(I think you can get it as flubenol) treatment. The new fish could too have parasites/worms and one should clear that as a possibility because although it doesn't kill them fast or often, it can lead to secondary issues due to the stress on the immune system.
Having said that, your new fish, as long as they're healthy to start with, are unlikely to be struck by the same thing. Its all about the immune system. Very clean water and good and plenty of food is the best way to boost the immune system of any newly purchased fish. Its the most crucial period.
I didn't mention it at first, but with that sort of soft water you have, have you measured how the PH fluctuates? Because although some fish come from very acidic water, their water is very stable. The Ph of acidic water in fish tanks goes all over the place. Wild discus, altum angels still need very stable PH. If your KH is very low, its is a certainty and it could have been the trigger of your troubles. Its just something I want to mention. Its a lot more difficult to keep acidic water stable.
However, if it were me, when getting the new fish, I'd give everyone a flubendazole(I think you can get it as flubenol) treatment. The new fish could too have parasites/worms and one should clear that as a possibility because although it doesn't kill them fast or often, it can lead to secondary issues due to the stress on the immune system.
Having said that, your new fish, as long as they're healthy to start with, are unlikely to be struck by the same thing. Its all about the immune system. Very clean water and good and plenty of food is the best way to boost the immune system of any newly purchased fish. Its the most crucial period.
I didn't mention it at first, but with that sort of soft water you have, have you measured how the PH fluctuates? Because although some fish come from very acidic water, their water is very stable. The Ph of acidic water in fish tanks goes all over the place. Wild discus, altum angels still need very stable PH. If your KH is very low, its is a certainty and it could have been the trigger of your troubles. Its just something I want to mention. Its a lot more difficult to keep acidic water stable.
