My son has a 5g hexagon tank that has 2 small koi fish in it that he has had for a few months now.........one that was won at a carnival and the other his mom got for him.......i was planning on moving them to a 10g so that they have more room.......is there anthing else that i could put in there with them other than other koi or goldfish.......the normal temp of his tank is around 73-75 degrees.....i just wanna add a little something else to it, because he love fish and the thing is he is only 2.