I am using a Fluval 88g bubble counter in my Co2 system. I use a 2kg FE, with regulator, needle valve etc, through the bubble counter, then a splitter into two separate CO2 lines, supplying two glass diffusers, each placed just below the inlets to each of my two Eheim 2217 filters to mix the Co2. I have a non return valve between the regulator and bubble counter to protect the regulator etc. and a needle valve in each Co2 line after the splitter to regulate CO2 flow to each diffuser. I have used this arrangement for a few years now, but have one minor issue which bugs me. No matter how little water I have in the bubble counter and no matter what pressure, a small amount of water from the bubble counter is sucked up the CO2 line/s towards the diffusers (I can see it moving up the CO2 tubing). I only need to top up the bubble counter every couple of weeks. So my question is, "is it normal for a small amount of water to go up through the CO2 line and Is water the best/only liquid to use in the bubble counter?" - many thanks