Meenens Happy Pets

I have a 20 gallon Fresh Water tank. It’s a new tank of three months that has finally settled down. The water tests come back perfect. I even double checked with a local fish store. There are four rasporians, one tiny tetra, two golden hardy starter fish (name unknown), one catfish and until three days ago two plecostomus who died within 24hrs of each other. Today, my perky catfish of two years looks sickly tonite. The other fish look ok, but I wondering what’s happening? I feed once a day, but had not included algae wafers to my bottom feeders. In death they looked okay, no sores , or such. One golden starter fish is now in a mini hospital tank, because it has suspicious red spot near its gill that gets better then worsens. I had thought it was ammonia poisoning or just a bite from the bigger goldy. He has two more days of treatment of gel tek for possible bacterial infection. The main tank is cleaned once a week, 20% water change with vacuumed gravel. I am at a loss.