PV=nrT.

It's the gas law. As you depressurize CO2 it's going to become colder... But it doesn't degrade the CO2. I mean, I suppose if you had really good control you could do a bubbler thing... but you know some people use dry ice in a cooler with a hole or 2 poked in the top for CO2 enrichment, and it's reasonably inexpensive.

I don't know how great the extra CO2 is for roots. Someone more into hydro would be able to answer that... My hydro experience has been DYI flood and drain... haw haw.

Hm. You get me thinking... If you want to do tank CO2 enrichment, and use the decompression as sort of a heat remover for the res, maybe you could run the output from the tank through a copper coil not INTO the water but THROUGH the water.