I have a 30 gallon aquarium, and have had it for a year. I only have 7 fish, because my fiance and I wanted them to grown big, and they have! We have 3 tinfoil barbs, 3 bala sharks, and 1 pleco (an algae eater, highly recommended to have one--they look really cool, too!)
When we bought the fish, the pet store people told us to buy them in 3s (these particular species) because they school together, and won't get aggressive with each other.
Maintenance is not too bad, I change about 6 gallons every other week. I use tap water, but put in tap water conditioner that I bought at the pet store. I leave the water sit in gallon jugs without lids for a week before adding to the tank.
I advise you to monitor the pH level, especially at the beginning stages. We had a drop in pH and lost 2 fish as a result. It is important to maintain a stable pH, there are things you can add to raise or lower it, but if the pH changes, those additions don't always help.
Fish are awesome to watch as they swim, and your cats will love them