Planted Tank Enthusiast
Join Date: Sep 2009
Location: Kingman, Arizona
Even one convict is way too much for that tank. They get 6 inches long at maturity, and pretty aggressive. They will start killing each other very soon, or your water will be so bad that it will kill all of them.
And a lot of people get told that they need fish to cycle a tank, but you don't. You can do a fishless cycle by just setting up the tank and 'feeding' it with fish food as though there were fish in it. You can also drop a little bit of raw fish from the grocery store in there (or shrimp, I guess). Cycling with fish is pretty cruel to the fish; you are giving them a poison bath that will at best damage them, but probably end up killing them.
I would recommend taking the convicts back where you got them as soon as you can, and get something more suitable for that size tank. (once it is cycled, of course!)