I have a 65-gal
tank with a sump filter below, which houses the protein skimmer and return pump. I have a hang-on-back siphon return canister with dual large-diameter return hoses routing the water into the sump filter itself. I use an AquaLifter siphon pump to suck the air out of the siphon chamber to permit return water flow. I also use a prefilter on the intake of the AquaLifter.
Recently I've observed the return pump sucking air due to low water level in the sump, and have had to turn off the pump, skimmer & Aqualifter to avoid tank overflow. I've experimented with new AquaLifter pumps, new prefilters, and replaced the tubing involved. None of these actions have had any effect, and I still cannot run the recirculating pump. When I start it up and watch the siphon chamber, I observe that the AquaLifter has sucked most of the air out of the siphon chamber, and return water is flowing, but apparently not at a rate adequate to match the flow of the return pump, thus causing the water level in the tank to creep up to the rim. The air bubble in the siphon chamber does appear a little larger than it used to be.
I'm at a loss with this system. When I test the several AquaLifter pumps (some brand-new), they seem to have strong suction. I've also run it briefly without a prefilter, and still had the rising water level, so it seems air leakage in the prefilter is not the cause.
Maybe AquaLifters are just not up to the job? Or maybe I have a crack in the siphon canister?
Appreciate any advice!