I know CO2 tends to degrade certain materials - will CO2 rich water impact a Hydor ETH if it sits directly after the reactor? I believe the innards are glass but worried that the plastic ends where the hoses plug in would degrade overtime - thoughts?
No, it only has a slight degrading effect when it is full strength as it is in the CO2 delivery tubing but after it is mixed with water the effect would be so minimal it would be difficult to detect. I've used the same Eheim tubing, Hydor 300 watt in line heater, etc. for years and if it was that destructive it would effect your canister, media, impeller, on and on but nobody would be using CO2. I've rarely seen it have a negative effect on tubing but after a year or so some will start to get brittle, but I had Tygon tubing on my tank for years will no ill effect and currently I use GLA's clear thick wall tubing and it seems to be holding up well but plastics have come a long way over the past ten years or at least I think they have.
I'd just recomend making sure your Hydor is vertical and not horizontal. The only concern in my mind would be if CO2 builds up inside the heater, and causes the bulb/element to overheat in the abscensce of water flow.
Very low chance of that happening, except maybe when you have your pumps/filter flow turned off.