Just sharing my experience here. Fully freshwater fish. Fine sand to me, is a must when keeping these fish. They will dig into sand to camouflage from prey or potential predators. NO sharp edged sand must be used for this purpose. They are largely piscivores, meaning they eat fish, live 1 especially. They aren't keen on ghost shrimps or any other types of shellfish. But u can train them to eat market prawns. A congo puffer kept in a sandless tank with no fish to eat is an unhappy puffer. The colour of the sand will determine the colour of the puffer-mine turned from red to a very light brown colour when in reef sand environment. Having a single fish in a single tank will do best to prevent injuries or death to any of the fish that you want to keep along the congo.
Last thing: Only if you have full commitment do u attempt to keep these fish. They demand very clean water and cleaning the tank MUST include cleaning of the sand. However, keep the tank environment with little changes as possible. Dirty sand with stress makes this puffer very prone to fungi infection
