Here's the situation with turning on/off your DIY co2. It is not bad for your plants to be exposed to CO2 at night, they just don't use it. In fact, plants will convert co2 into malic acid at night, storing it for use the next day, so if anything, it's good for them. However, it's not that much better for them than turning off the co2 at night. The bottom line is, leaving your co2 on at night wastes co2. Now, for a pressurized co2 system(i.e. out of a gas cylinder) you can just stop the co2 and it's just cut off, no problem, saves money.
With DIY CO2, when you "turn off" the 2L bottles, the yeast keep producing, and the pressure just builds inside your bottles. People will say this is "dangerous." It's not. As long as your aren't using glass bottles(average glass can withstand up to 5 psig of pressure, i.e. not a lot) you won't build enough pressure in a single night to bust your plastic 2L bottles. However, what happens is, the next day, when you "turn on" your co2, it doesn't just trickle out, it "explodes" out, sending all that co2 up to the surface where it is wasted.
On top of that, because you turned off your co2, now your co2 level in the tank is very low(co2 in tank been diffusing out at the surface all night), so it has to build back up to a useful level.
Point is, with DIY CO2, better to leave it on at night.
