Help, pH not going down.

[Ng Dong Wei]

Hello, I have an issue with my high pH (~7) not dropping despite having a fairly established system. Here are the specifications of my system:

 

Hybrid DWC and ebb and flow w/Hydroton

~300 gallons at ~99 three inch tilapia, they are fed on fish pellets

DWC is ~1.5 times media bed

Currently, there are approximately 50 plants growing ranging from lettuce and basil to tomatoes and peppers.

 

The treatment for a week prior to this incident was the following:

0.25 teaspoon MgSO4 and CaOH and a few drops KOH.

 

What was worrying was that despite adding bases to the solution the pH reading was in the 6.4-6.6 range. Furthermore, my tomato plants increased in health but the new leaves in my more mature basil started to develop choloris and necrosis (yellowing and browning). Young basil transplants did not display any of these symptoms. In addition, while my lettuce plants looked healthy and growing well, their leaves displayed strong curl downward. Nitrates were at >80ppm, nitrites were near zero, and ammonia was also low.

 

As of recently, I have stopped the treatment of adding the bases.

What do you guys think might be happening?