I have a 20 gallon freshwater tropical fish aquarium in my room. It's been there for about 10 months and I've not had any problems that are as bad as this one. I have had the same gravel, blue and green, in the tank for the entire time I've had it. But today, I decided I wanted a change in color so I went to buy white gravel. After doing so, I started to clean out the tank. I got my usual containers, filling them with the water already in the tank before putting the fish in so I could continue cleaning. I took out all the decorations and stored them away except for my bubble blower. I had to take out all the water and the blue/green gravel, giving it a thorough wipe down without soap to get all the old gravel out. Then I rinse, sifted, and put the white gravel into the tank. After that, I filled the tank back up as usual, put the filter in, turned the heater back on, made sure the air compressor for the bubble blower worked, and conditioned the water with AquaSafe. I then put the fish back in, but noticed one of the fish had a red sore on him. I then took the filter out and put MelaFix and PimaFix in the tank so that they would work without getting absorbed by the filter. I had about nine fish then. Well I went out of town to get more fish and new decorations, came back, and saw one fish dead (not the one with the red sore). I immediately took him out, but noticed some others were acting strange as well. They were sitting at the bottom or just staying in one spot. Then the fish with the red sore died, as well as another one very soon after. Now it looks like two more are about to see the same fate. What happened? Was it the gravel, or was it the open sores infecting other fish? I'm very confused because I've never had this problem and I did everything exactly the way I've always done it aside from change the gravel out. Help as soon as possible please