If you don't have one, it won't break and you never have to
worry about it. Not that they are espeically prone to
breaking. By not having one you won't hurt your fish or
plants. using one on the timer with you Lights, you cut off
the addition of CO2 during the dark period when the plants
don't really use it. This will save you some CO2 in the
long run so fewer trips tothe refill station. Otoh, if you
don't have on, your pH will proably only swing 0.2 units
between highest and lowest points, so what's the big deal?