I have had a tank for about 4 years. I noticed my fish becoming very inactive even though a few days prior they were swimming around happy as could be. I phoned the pet store asking about potential issues in my tank. The asked what kind of filter I had and I told them i had a Hang On Back filter and that I had recently replaced the cartridge. The store (whose advice I rely on and trust a lot) suggested I get a filter that doesn't take cartridges. What kind of filters would you suggest? Thanks in advance
