A single pair of convicts should have a 25 gallon tank. All they really need is a cave - this can be done with flowerpots, pvc pipe, rocks... the choice is yours. Sand or a fine grade gravel is idea, plants are optional, but will probably not last if the aquarium isn't big. The minimum size tank for two pairs is 55 gallons, and people have found that even this tank can be too small - these are very, very territorial fish when spawning.