I have cat litter in my 55 gallon tank, been almost a full year now, and I love the results from it. I use a specific brand called
Special Kitty: All Natural Kitty Litter - Find it at Walmart for a couple $.
It 100% all natural clay, doesn't crumble like other litters, and has a nice high CEC rate. Never effected my Ph, haven't lost a fish, or shrimp because of it, it's very cheap, and you get a lot for low the price.