Pressurized CO2 question

I have a pressurized CO2 system setup for a 75 gallon. It contains a 20 lb CO2 bottle, regulator, needle valve, then pressurized windshield tubing running from the needle valve to a bubble counter, then more of the tubing running to an inline, homemade CO2 diffuser.

The CO2 diffuser is hooked into the output line of an Eheim 2128. The way I connect the tubing onto the diffuser is from the plastic piece from an old air pump that I sawed off stuffed into a small drilled hole in the side of the pvc pipe.

I'm worried about the water pressure pushing itself into the tubing. I know that there are numerous others that have similar set ups and was wondering what anyone's experience was with the CO2 pushing into the cannister from the tube and the water pressure pushing any water up into the tube.

Is this a problem? Or does the CO2 pressure keep any water from entering the line? If not what can I do to prevent it.

Thanks.

Where exactly will the brass check valve need to be attached? And does this mean that, yes, water will push itself into the CO2 output tubing? Any experience that you can tell me about this?