Do you have a glass or top on your tank?
What time of day are you testing?
Do you do regular pwc
Do you have any power heads for extra flow?
Is your water surface agitated or smooth?
Is it's smooth, get a power head or two a/o aim your returns so that the water surface is agitated. That is where gas exchange takes place, removing CO2
from the tank and adding Oxygen. A buildup of CO2
can cause low pH.
Partial watter changes should be all you need to keep it stable, once you get it in the proper range. Overfeeding and excess nutrients will cause it to go low. Also,
pH will vary during the day, being lowest in the early am and highest late in the day. It should be in the 8.0 - 8.2 range.