syphon malfunction??

are air bubbles accumulating in the siphon tube?

how strong is your return pump?

what does "very strong" mean?

are you flowing at least 500 GPH through your display tank?

Is your pump rated at 700 GPH or are you certain that your true return is 700 GPH? If your pump is rated at 700 GPH and you have a 90 degree angle or two that will drop it significantly. Also, it may be 700 GPH at 0' head, you'd be getting much less real turnover at 4 feet head.

the only way to fix it would be to run more water through the tube.

if you drill a hole in the top of the siphon tube your going to get even more air in it, even if you have a powerhead sucking the air out.

slugstheone,

I had a similar problem when re starting a syphon there was a bubble in the very top of the up side down U and when the the bubbles came thru from the water falling into the skimmer box it added to the bubble until it no longer pulled a good amount of water which would then have lead to an over flow if i had not been home. There should be no bubbles at all stationary in the tube. The key for me was to stick the small tube so far up the back of the syphon into the U that it had to pull the full amount when started. This is a pain to get the starter tube back out but i have never over flowed (Knock on wood). Also if you have the wrong end of the tube in the front (inside tank overflow) which should be the shorter end the tube, it will touch the bottom of the overflow or so near it that it would severly decrease the intake. I also have had snails crawl up the tube which is unpredictable and probably not your problem. It may sound dumb but i lift the syphon and tap it on the parts it hangs on and never let it stop sucking (leave the water in the overflow at least in the front) to get the bubble moving and on the little ones it usually works.

Good luck