MY 55 Gal has been running since late september. 4 weeks ago I got 6 new otos, one of them got sick (some sort of white foam was growing on its tail, doesnt seem to have spread to any other fish) and died 2 weeks ago and another one died yesterday. I didn't make too big of a deal of it because I know that otos have a high chance of dying after being moved to a new tank. But today I found one of my 6 Corydoras Pafeatus dead, couldnt see anything unusual on its body. I have about 50 fish in there, most of them small like danios and cloud mountain minnows, the only fish that have the potential to grow are my boesemani rainbow, siamese algae eater and my bristlenose pleco. Tons of plants and a good filter so oxygenation shouldnt be a problem. The only thing I can think of is that this morning I placed a cantalope with bug sticks in it on a fork and placed it in the tank for my pleco soon after showering and there may have been small amount of soap on my hand and arm, i usually do this in the evening but I was in a hurry and couldnt feed them so I figured this would last them for the day. I also started adding pottassium supplements in my tank, 2 corks of it every 2-3 days before my oto and Corydoras Pafeatus died.