The macro algae will take in CO2 and release 02 during the day. But during the nighttime, they will respire, meaning they take in O2 and release CO2. The CO2 produces carbonic acid which will lower the pH.

So you don't want to drip anything at night and you don't want to have a sump ... the only two things I could think of would be: get rid of the macro or get one of those hang on fuge things to run at night