I am very new to fish and aquariums, I recently came across a 55 gallon tank and decided to put it to use. I started with 3 goldfish in a 10 gallon tank for about 9 months. once I got the aquarium, I transferred the goldfish into it. after about a month I got 3 Oscars or potopoums (not sure of the name) bottom feeders, 1 koi silverish, 2 black goldfish, whom all died within a week of putting them in the tank. The goldfish became either stressed or sick with white spots, upside down floating, bloody fins and ****, gasping at the top for air. swimming with weakness. I did a complete water change and tank cleaning. I also boiled the gravel rocks to remove any ammonia or buildup. I recently purchased 2 baby bala sharks and I put them into my 55 gallon tank about 3 hours and a complete water change after they came from the store. I also purchased 2 Iridescent sharks as well. after putting them in the tank the Iridescent sharks divided themselves and stayed on opposite sides of the tank just floating in one spot and hitting the glass. The Bala Sharks stayed together but in one spot on one side of the tank with 1 of the iridescent sharks. I noticed some spots on one of the iridescent sharks and I put some stress zyme and metafix into the water. the fish continued to swim in the same spot and way for the next day or two before turning upside down floating and gasping slowly. I would really like some help on maintaining a 55 gallon aquarium and taking care of fish. I would also like to start a community tropical fish aquarium. any help. I was thinking of getting cherry barbs as I heard they were good starters for aquariums.