help, shit hydo hard !!!

first of al people i have 2 nft tanks and the ph keeps droping for 6.1 down to around 4.7 sometimes lower is ther any reason for this ? my best 20 babies have brown rust spots on the leaves and dont seem 2 healthy as the electric been of for 2 days ?

my pH always goes up from 6.5 to around 7.5 in a day and up past 8 after two days. seems weird to me too, certain products will continually change your pH though. like my Canna Rhizotonic is a pH up, and it seems to always rise when i have it in my res