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.