eds said:
Some people do just bubble CO2 into their filter intakes, but, after talking to the Ehiem supplier in the UK he was aghast that people would do that and said that in his opinion he'd have to think carefully about potential harm to the filter if they had been intentionally adding gas to the canister!
this is mainly because of the worry that the co2 will degrade and ruin the rubber seals in the unit, however.. many people have tried adding co2 directly into the inlet (including me) and seen no ill effects.. its always worth rubbing your filter seals in silicon grease anyway when you clean them out.. if you do that, i cant see what harm it would do.. after all, theres always co2 in the water anyway, no matter how you add it to the tank, the only issues would arise if there was co2 collected around the seals, which would be highly unlikely, if possible at all.