Ph Has Dropped Dramatically

[mozza7avfc]

Hi ive just tested my water and my ph is 6.4, a yellowy greeny colour, why would this happen???
Whay can i do to get it back to normal in th 7's???

I need to do a water change anyway now as ammonia has crept up, would this increase in ammonia be the cause??

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
How long has the tank been set up.
What substate do you have, do you have any rock or bogwood, What ornaments do you have in the tank.
What your tap ph.