[reef master 25]

Hello I'm still new to the hobby but I'm having a very bad time with either cyano bacteria or diatoms I'm not sure which it is. My tank has been running for about 8 months now so I don't think it's diatoms but my p.h keeps bottoming out but my nitrites nitrates ammonia alk. Is all fine my alk is at 8 all other levels are at 0. My p.h at the moment is just under 8 and that's where it stays most of the time. But just recently it would not even read on the test it was so low but yet everything else was ok. I have a 75 gal. With a hob reef Octo 100ss skimmer and a hob filter which I'm fixing to get rid of and get a Br gfo/ carbon reactor but I can't get rid of this bacteria/algae please help I'm going crazy.not letting me post pictures