Well, the parts of general fishkeeping apply, a tank the right size and nice clean water, but DIET is one of the most important factors. Guessing your "common" would either be a Hypostomus or a Pterygoplitchthys they need a good deal of roughage, fresh veggies with some meaty foods such as worms too. When younger they do well with more veggies, but tend to want more protein packed foods as they age. I'd feed a 75:25/70:30 veggie: protein diet. Best to feed at lights out as they are nocturnal fish.