I've had my first tank about a year now so I'm still a bit new to the hobby, Over the past three days I've had two cory catfish die on me, one was showing signs of what I thought was swim bladder disease because it was swimming erratically and then it would just stop and sink sitting on the bottom for a while on it's side or back, so I moved it to the hospital tank and administered aquarium salt. the next day my smallest cory died overnight so I found it upside down not breathing and unresponsive to touch. Meanwhile the one in the hospital tank seemed to be doing better, it was swimming better and staying upright but then this morning I found it dead as well. My water parameters are good 7.0Ph, ammonia, nitrates, nitrites are 0 or really close to 0. the only thing I've done to the tank recently other than the usual 25% water change is I added a sponge filter from the co-op, a white stone decoration that was rinsed and scrubbed with a brush thoroughly, and a little ceramic elephant decoration. I'm not sure exactly what I'm doing wrong if anything.