If you keep a Chaca chaca it should be the only fish you keep in there, they either eat tankmates or are unable to compete with them for food.
You can keep a single Chaca in a 18x12x12" tank with a couple of inches of sand and a flat rock, they are one of the few species of fish that actually benefit from less room as it makes it easier for them to feed as they wait for the food to come to them and do not actively hunt.