I have a 75 gallon tank that pumps up to two barrel grow beds. Each of these has a bell siphon made from 1", 2" and 3" pipe. I have airline tube coming out of a 2" cap. I basically followed the design that I saw online. One of them works like a charm but the other one runs continuously, returning water to the tank all of the time. (They are identical in construction

I have removed all of the hydroton from the grow bed and looked at the siphon but I can't honestly tell what is going wrong. (I already did this twice!)

I also tried changing the water flow and removing the elbows at the bottom. These changes make the siphon work for a short while but inevitably, it reverts back to continuously running water.

Any suggestions of what I might to do fix it?