Your tank seems stocked enough. Remember, general rule of thumb: One gallon for every inch of ADULT fish. Even though your fish may be small, they will get bigger. Neons and Zebras get to be about 2 inches, so thats already 26 inches, 26 gallons and then you still have 4 more fish. if you add fish, your water quality will degrade and result in unhealthy fish and eventually fish deaths.