29 gallon Biocube!

[Aysha]

My Ph keeps crashing does anyone have any ideas
its done it 3 times since yesterday.
I had to treat the tank with ich attack for the last 2 weeks JOY!
and thats the main thing that's changed.

I've done 3 15 percent waterchanges in a week and a half .
yesterday after I finished one I noticed my chromis acting wierd.
Ph 7.2
added marine buffer and marine builder
ph 8.4
checked again today
ph.7.8

ammonia zero
nitrates zero
nirates zero

I feel like im a really really bad fish keeper
ANY ideas whats causing this

29 gallons
temp 27
35 lbs live rock
15 lbs live sand
various inverts
2 fish
salinity 35