My system is 6 months old and the pH has been dropping since day 1! I can't figure it out and it's driving me bonkers. No dead fish, no dead spots, I even replaced a bed that had pond-liner cuz I thought it might be the culprit. Nope, still dropping! It wants to go below 5! Plants seem happy, I get great squash but mushy tomatoes. And a gigantic pumpkin that is very happy. Fish seem fine as they…