Chilli raspora

I’ve had a bit of a disaster, my tank has been cycled for 3 weeks tested literally everything with the api test kit and the gh and kh, so I got 5 galaxy Rasbora and 6 chilli raspora, my galaxy’s are doing very well they seem very happy, but my chilli raspora’s 😬 3 got killed by my filter and it’s a tiny filter, 1 was dead just after I put it in to the tank, then about three days after one of them is swimming funny it’s as if it doesn’t have its side fins I can’t see tho because it’s so small, it goes head down quite often, I have put it in a separate tank. Could they have all had some illness?