Ph of fish tank

[Earl Phillips]

The Ph in my 350-375 gallon hot tub tank continually runs around 7.8 even after I put in 1ml per 50 gal of ph down pond treatment every day. The fish are doing good except catfish. The plants are producing flowers and fruit. This is a pump and drain system that cycles pretty regular unless a pee gravel rock gets involved. Its been producing nitrate for at least 80 days. I clean the bottom of the tank daily by siphoning with a garden hose pipe 3-4 gallon of waste water off the bottom well of the tank and feed duck weed with it in a separate IBC for that purpose. I have noticed dead worms in the tank as of late. Could these two things be connected?. I replace the waste water with fresh rain water that tests 7.0. Anyone got any ideas whats up?