What's lowering my PH?

[MithrilRider]

My tap water's PH is 7.2 but in my tank it goes down to 6.5. The only organic things I have in it(besides the fish) are some rocks and plants(crypt, java moss, dwarf onion, aponogeton crispus). I took out all the rocks and left them sit in a bucket overnight. In the morning the PH was the same. I'm not sure what it could be. Any help is appreciated.

 

[Paul_MTS]

test the PH at a few different times during the day.

it may be falling due to your plants creating CO2 at night which lowers PH. if it rises during the day this is because your plants are using the CO2 and producing 02.

 

[phishluvr]

yep, it's the plants.