Couple things here:
1. if you're gonna create a vaccum with a
DIY setup, use very thick plastic, or glass, juice bottles. a vaccuum will collapse the pop bottle eventually. the risk is always there.
2. never EVER restrict the flow of a
DIY setup from venting the
CO2 it creates. I can adn WILL explode when the pressure builds up. I can't tell if the brass valve is being used to restrict the
DIY, but if it is, remove it. Again, the risk will always be there for it to literally explode if you cap the pressure.
3. to answer your question, running extra
CO2 when you don't need it is just a waste of time and effort.
DIY rarely hits lethal
CO2 levels, so you'd basically have plenty of
CO2 in the water, which may benefit the plants a little, and will definitely lower your pH.