I've had my tank up & running w/plants for about 2 weeks now. 10 days since I put the fish in (3 small barbs). Ammonia is presently at 0. pH is reading between 6.6-6.8. Do I need to bother with the high range pH since it only seems to apply to 7.4 & up? Also, now the nitrite levels have increased somewhat - about 2ppm. Just did a 25% water change. What level do I need to keep it at in order to not endanger the lives of my fish?