I think what would be best is keep everything you have, add 4 more phantom tetras, and add 6 more cardinal, neon, or black neon tetras and call it a day. If you calculate the "inch per gallon rule" you'd get 17 inch's but the reason it would work is because the tetras are bottom, middle, and top swimmers, they use the whole tank. As the apisto only uses the bottom of the tank, you only see him at the top when there's FOOD! If all of the tetras were only bottom swimmers, only middle swimmers, or only top swimmers I would not recommend this.