+1 to everybody. You need to upgrade them sooner rather than later. As you know, they create a heck of a lot of waste, and even with lots of water changes you aren't going to be able to keep the water clean enough. This will stunt their growth.
Look on your local classifieds, craigslist or whatever, for a second-hand 55 gallon. You are going to need a very powerful filter, too. I would also suggest getting a heater - orandas, from memory, like to have their water at about 68.