water changes

Question
Hi i have a 27 gallon cube aquarium which has one angelfish about 4 inches. I do two 20% water changes every week.My water perimeters are always ammonia 0. nitrite 0.nitrate 5.0 i also  clean the gravel at this time. ph is 7.5 i know that is a little high for angelfish but he is doing great.
i read on several sites that a 40% water change is to much for a 27 gallon tank.
as i understand it the bacteria don,t live in the water column anyway. do you think i need to drop to 20% a week or stay at 40%.
thank you for your time.

Answer
In a 27 gallon aquarium, once a week would be better, it gives more time for good bacteria to grow between water changes. Also if you want to drop your p.h. the natural way, put drift wood in your aquarium.. Tanic acids from the wood make ideal conditions for angelfish.

hope this helps