I'm new to semi-aggressive fish and I'm not sure how they should be kept. I have a goldfish that is aggressive. He's not that big but he killed the two other goldfish that were brought home with him. He also killed my friend's goldfish Derrell who was almost twice the size of my fish. The tank has always stayed around 72 degrees(that's without a heater heating it) so I put a adjustable heater in it and set it to that temp. I always put heaters in and set them because our house heat has a tendency to go out and I've lost entire tanks. I know 72 is warm but I moved the tank around the house(which stressed the fish bad) to try to find somewhere it didn't get so warm and I tried putting ice cubes in it but that also stressed fish. Plus I have to buy ice(well water not good) and it was very expensive and the fish seemed fine in the warm water so I quit trying to keep it cool and just fixed it to where it stayed 72. I was going to try to find someone to adopt the goldfish because it's a 100 gal and I wanted to put more goldfish in when someone on a forum suggested I get a semi-aggressive fish since my goldfish is so aggressive. The tank is over filtered(like a goldfish tank should be) so I decided to give it a try. I got a baby albino rainbow shark. The fish get along great. My only thing is can I get other fish? If so, what kind would go with evil goldfish and shark? DO I have to just keep them and only them? Any advice on where to go from here would be great. My plan is to get a bigger tank and move them to it so there will be more room for other fish but what other fish can I get?