My daughter has a 7 week old 10 gallon tank with 2 platy's, 6 neons and 6 glo-lite tetras. Current ph is 6.8. Ammonia is 0, nitrites 0, nitrates at about 5ppm. The ph seems to creep up daily to about 7.4. I've been bringing it down with a single dose of ph down ever 1 - 2 days. What can be causing the daily rise of ph? Any suggestions on how to keep it stable?