I have had my tank running for 3 weeks, 2 weeks with fish. So far have 2 small angels, 2 blue gouramis and 2 bristlenose cats. ! have an aquaclear 70 power filter. Ammonia 0, Nitrite 0, pH 7.2, Nitrate 5ppm. When should I add more fish. how many and any suggestions as to compatible species...