This is my first post, am looking for advice as a new fish owner. Some background, I had 3 African Dwarf Frogs for about 5 years, lost 2 of them to dropsy late last year, the third has survived and is doing well. I thought it would be nice to get him some friends so I bought 3 leopard cory catfish last Saturday. Water change was done prior to putting the catfish in. On Tuesday I noticed one catfish at the top of the tank, after some googling I thought I'd do another partial water change. I kept noticing this behaviour on Wednesday so did another water change and another on Thursday and Friday. On Friday one of the catfish was lying on its side and couldn't swim properly, I thought it might be a swim bladder problem. Unfortunately I don't have a quarantine tank so couldn't move him out, continued with daily water changes, lowered the water level and stopped feeding. Yesterday all three catfish started lying on their sides. One was noticeably more active than the rest but still drunk-looking. Did another water change. Unfortunately overnight two of the catfish have died. The remaining one switches between being pretty active but unable to swim properly and lying on its side. I've been checking water parameters daily, no issues apart from the water hardness was slightly lower than expected so I added some epsom salts yesterday (also read this might help relieve symptoms of swim bladder?). I still haven't fed them. The filter is an Interpet (unsure of the exact model) but has been in there since last year so definitely cycled. I think I may have overfed them. I feed the ADF defrosted bloodworms so decided to feed these guys the same. As I've not had them before I fed them every day from Saturday to Thursday to try and work out how much they ate. During water changes there was a lot of uneaten food. Tank size is approx 20l. I am in the process of cycling in a 50l tank. Another thought was maybe I overstocked the tank so I hoped the daily water changes would help until the new tank is cycled in. Any advice to help me save my remai catfish would be great. I feel really guilty that I've somehow managed to kill these fish in less than a week.