Okay so I had five Tiger Barbs in a community tank. They killed two of my fish. I moved them into a smaller five gallon hexagon tank. Four of them killed each other. I found them at the top belly up with no fins at all and bites taken out of them. So there were two fish left a Spotted Green Puffer Fish and a Tiger Barb. I cleaned the tank two days ago and put all of the necessary chemicals in for making tap water safe. My tank was full of algae. So full that the bottom of the tank looked like it had grass. So, I did a 93% water change. I then returned the fish. Oh I forgot to mention the Green Spotted Puffer Fish was dead when I went to clean it. My first fish. 
Anyway I returned the Tiger Barb and tonight when I went to give them before bed meal, I found him floating upside down with gills still moving and he looked perfectly mobile. So I hurried and moved him into my community tank and after about thirty seconds of upside down floating he was right side up and fine. What was wrong with him or the tank or both??
Okay so I just found out that for some reason my heater wasn't working and the water was ice cold. I did have the Spotted Puffers in a 38 gallon bow aquarium but it went downhill with a disease so I had to bring my spotted puffers into my emergency stand-by 5 gallon tank I know it was torture to put all of the fish into such a small tank, but this all happened so unexpectedly. I would never purposely put all of those fish into one tank. I guess I should told you all of this in the first place I apologize. I have the heater back up and running and I want to let it warm up until about 4:30 p.m. tomorrow. My main concern is the same as why I moved the Tiger Barbs in the 5 Gallon Hexagon tank in the first place: Will they hurt my other fish? I have: One African Brown Knife, Five Rainbow Sharks, Three Bala Sharks, Three Angelfish, One five inch Plecostomus, Four Giant Danios, Four Bloodfin Tetras, and now apparently one Tiger Barb.
Anyway I returned the Tiger Barb and tonight when I went to give them before bed meal, I found him floating upside down with gills still moving and he looked perfectly mobile. So I hurried and moved him into my community tank and after about thirty seconds of upside down floating he was right side up and fine. What was wrong with him or the tank or both??Okay so I just found out that for some reason my heater wasn't working and the water was ice cold. I did have the Spotted Puffers in a 38 gallon bow aquarium but it went downhill with a disease so I had to bring my spotted puffers into my emergency stand-by 5 gallon tank I know it was torture to put all of the fish into such a small tank, but this all happened so unexpectedly. I would never purposely put all of those fish into one tank. I guess I should told you all of this in the first place I apologize. I have the heater back up and running and I want to let it warm up until about 4:30 p.m. tomorrow. My main concern is the same as why I moved the Tiger Barbs in the 5 Gallon Hexagon tank in the first place: Will they hurt my other fish? I have: One African Brown Knife, Five Rainbow Sharks, Three Bala Sharks, Three Angelfish, One five inch Plecostomus, Four Giant Danios, Four Bloodfin Tetras, and now apparently one Tiger Barb.