I hate to tell you, but that is a standard 10 gallon tank. Many people keep gold wonders in similar tanks, but you will see more natural behavior if you get something smaller. While not a lot of people see them max out they are pretty large when they do. You have some other great options if you like killis. Pseudepiplatys annulatus is beautiful and will do great in there. I really cant think of a nicer killi. You could do Epiplatys dageti. They are pretty tough for their size. Another nice one is Aphyosemion australe the gold version is very nice and fairly available. If you just want something vicious it is hard to beat Gambusia holbrooki, but I would stick with the killis