not any fish, there are some cats who will grow far too big for your tank such as the Red Tailed Catfish and some catfish who in my opinion would be too small to keep with a Brown KNife-fish such as Otto cats.
what he saying is buy some cheap fish, maybe 6-10 neon tetras before any other fish and leave them in there untill the water in your tank has finished its natural cycling. look it up online to find out what to do in more detail.
if they don't have them at petsmart you can always order online, www.liveaquaria.com, check that sight out. it's the only place i look online when ordering fish and ive never had any problems.