i'm stumped
 

my drain for my tank has been working great for years(stockman);but in the last few months it hasn't been draining properly.some days(most of the time)it drains properly and i have a fall of 4" from the top of my over flow ;other days my water level is an inch above my overflow.my water level is usually 2" from the top of my tank as i use a wave box-but on the days it doesn't seem to drain,its about 1/2 " from the top.i checked my drain and it's clean with good water flow.any body know what is happening(besides full moon;though it does happen then)or how to fix the problem without adding another drain as i dont want to tear it apart(220 gal peninsula)

If the water level is rising in the top there is either a clog of some kind or the return pump is pumping faster than the drain is channeling somehow.

What are the dimensions of the plumbing and what size return pump are you running?

stumped
 

my drain is a 2" stockman and my pump is a bayside poseidon ps2 going into an oceans motion wavemaker with 1"return lines(4).my drain is clear as i just inspected it today.i have actually turned down the flow leading to my pump(ball valve)the thing is it doesn't happen all the time.maybe 2-3 times a month for a day or two

You might have bacterial build-up on your drain pipes. Under certain conditions, like carbon dosing, etc. the bacterial mat swells reducing the available section of the pipe, usual somewhere were you already have a flow restrictor.

stumped
 

drain is clean-i put my aquamedic scrub brush down last week and cleaned the lower drain as well

just a shot in the dark but is it possible your pumps pumping out more than it should at times?? maybe somethings in its messed up

stumped
 

no idea-only happens 2-3 times a month.i was thinking it was some type of vapor lock

I used the same pump in my experimental set-up at work. I've noticed the flow rate can vary. For me what happens is that small air bubbles get into and build up int the pump which reduces output. Then after a while I get a big bunch of bubbles that comes out of the pump and the Flow rate bounces back up again. This has been throwing my automatic control because the control system tunes itself to deal with the gradual decrease in flow and is then thrown out of whack when there is an instantaneous change in flow rate as the bubble leaves.

Do you hear your pump getting noisy at all?

a quick easy fix would be to try a different return pump if you have one lying around or could borrow for a day, im having similiar issues and im thinking its my pump.

im getting exactly as scubasteve described as well.

stumped
 

pump has always been quiet and to change the pump means draining the sump partially and a rebuild on my stand.maybe its one of those little mysteries or it could be a full moon.thinking about a double drain but it would still run into the same bulkhead as i have no room in my overflow for another bulkhead and i don't want to drill my tank again