Hi. I have an established 20 gallon and I have switched out filters quite a bit in the last two weeks. I have been having issues finding one that works and is quiet. I finally found one and it doesn't fit my tank without the water level being about 2 inches too low. I have an underwater filter in my other 20 gallon that hasn't' been running long either. That tank has had multiple deaths, but I think it is because my bf put new fish in without letting it cycle. Should I be able to just switch the filters?