I have a 5.5 gallon tank with one 2" Apisto Regani. The tank is lightly planted and filtered with a sponge filter. I change 30-40% of the water per week. Would the tank be overstocked with a school of six neon tetras (which I already have) or black neon tetras? Could I even add a small, slow catfish like a kuhli loach instead/in addition? Other options for acceptable tank mates are greatly appreciated. Thanks!