I think if I were you iwould see about trading him back for a pictus catfish, they very seldom get bigger than five inches and are peaceful with fish to big to eat.
Either way here is the basics on setting up a small tank. Add your gravel, rocks, etc. fill the tank with water close to the temp. you are aiming for and dechlorinate it. Plug in your heater, you will need one. add your filter, if it is a hang on try to get hold of some sludge from a working filter in a healthy tank and add that to the media, add any plants you are wanting.Let the tank sit over night, checking several times to adjust the heater. The next day double check the temp. 78-82 is good for most tropicals. Add one fish and let it settle in for several days, if you have a test kit (recomended) test your ammonia and nitrites daily when they read zero start slowly adding more fish. A 20g would support a pictus, a couple corys, an opaline gourami, and 3-4 giant danios or congo tetras.
I really do advise trading off the iridescent shark (Pangasius sutchi) within the next few years that would require a pond 2 1/2' deep, 8' wide and 12' long.
Either way here is the basics on setting up a small tank. Add your gravel, rocks, etc. fill the tank with water close to the temp. you are aiming for and dechlorinate it. Plug in your heater, you will need one. add your filter, if it is a hang on try to get hold of some sludge from a working filter in a healthy tank and add that to the media, add any plants you are wanting.Let the tank sit over night, checking several times to adjust the heater. The next day double check the temp. 78-82 is good for most tropicals. Add one fish and let it settle in for several days, if you have a test kit (recomended) test your ammonia and nitrites daily when they read zero start slowly adding more fish. A 20g would support a pictus, a couple corys, an opaline gourami, and 3-4 giant danios or congo tetras.
I really do advise trading off the iridescent shark (Pangasius sutchi) within the next few years that would require a pond 2 1/2' deep, 8' wide and 12' long.
I got the irid shark in a 20 gal heater going, undergravel running and I put in some bio starter fluid stuff to rapidly mature the aquarium, he seems to be doing great and loving this new space! 
No thermal shock this time, though the temp right now is a bit below 70 it was rougher in what he was in. I have the other smaller guys ( two strawberry tets, four gups, four neon tets, small algae sucker and two cherry barbs) in a 2.5 gallon, and they also seem to be doing ok. They now have a little cheap filter/airstone which is good cause there are alot of them in that small space. We are getting a free 10 gallon tomarrow and I was wondering if I should even transfer the little guys into that tank. Oh and the shark will be getting two new tank mates as well since they do better in schools when they are young. 
I think I finally got this under control.
