Next take a small tiny powerhead, and have the out put of the DIY
snip
Plug the powerhead into the lighting timer.

When the lights come on and the plants use the CO2, the gas gets
dissolved and adds it to the tank. When the lights are off, no CO2.

I am doing exactly the same but instead of powerhead I am using a Duetto 50
Filter. This filter has a good air and water mixing mechanism. I am
injecting CO2 in that. This serves multi purpose i.e Mechanical & Bio
Filtration, slight extra circulation and CO2 diffusion.

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Thank You

Dominic
http://www.dlink.org/aqua