There are float valves made for the tubing size of the R/O unit I think 3/8 inch. I got mine from Discus Madness website when I was first starting with discus. He has good prices on equipment. It is cheap less than $20.00. I drilled a hole in the back of the tank, it has a flat side because it is a "rain barrel". I comes with a nut to fasten and the tubing just plugs in. It has a small amount of adjustment with a wing nut. Drill the hole near the top of your barrel.
Bill