Thats a large ph swing. Have you tested your KH yet? You should try to bring up your KH to between 3-5. If its already that high, you are probably injecting too much CO2. Try to keep your ph above 6.6. There aren't too many fish that like it much below that. The PH drop probably happened over a period of a couple of days. But when the CO2 runs out it'll rise rapidly and that might not be the best thing for your fish, Try to stabilize it a little by buffering the water or reducing your co2 production or both.
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75 gal heavily planted,50/50 Black beauty,Eco-complete substrate, Pressurised CO2 with solenoid, ph controller, AB Reactor 1000, 330 watts 9325K GE PC lighting,Ehiem Liquidoser, 2-Filstar xp3 canisters.