Why is my Durso overflow "stalling"?

[blackstallion]

I have a Marineland 225 with dual corner overflows plumbed in their standard Durso configuration, including hole with small vinyl tube on top of drainpipe cap for full siphon break.

Last couple of mornings when I come down to view the tank, I have discovered that one of the overflows is "stalled" and not draining, pushing all the water to the other overflow (which thankfully does not stall), although with approximately 1000+gph being pushed to one overflow, it has difficulty keeping up.

After I "reboot" by turning off the return pump, allowing the water from the overflows to drain into the sump, everything goes back to normal with both overflows draining properly.

What is causing this? If the overflow was clogged, I would not expect it to go back to draining normally after turning the pump off, draining, and back on.

I understand the small hole on the top of the drainpipe is a siphon break, could it be implemented wrong and is somehow causing me issues? If the water level in the overflow rose over that small hole, would it cause it to "stall"?

Also have to mention that the overflow that "stalls", also tends to vary the water level and will "flush" often.

 

[WVNed]

It's more likely that the tube is plugging up trapping air in the drain and air locking it.
When it stops try poking a fine wire or something into the little tube all the way through and see if that doesn't restart it. They get blocked with salt.

 

[blackstallion]

It's more likely that the tube is plugging up trapping air in the drain and air locking it.
When it stops try poking a fine wire or something into the little tube all the way through and see if that doesn't restart it. They get blocked with salt.

Thanks, I actually cleared out the tube a couple days ago and still the same.

I edited my post after your post to mention this overflow also "flushes" often due to varying water level.

 

[WVNed]

Those are all symptoms of air getting stuck in the drain and not being able to come out of the little tube.
It flushes when the air comes out the bottom and clears and it becomes a full siphon until the level in the top drops and it gets another dose of air.
Then the cycle repeats.
Is the little tube poking through the elbow a ways inside? Perhaps you need to pull is out a bit. It may be under water inside the drain.

 

[N.Sreefer]

My hob overflow always seems to stall on one of the tubes so I feed an airline tube into the pipe that's stalled and suck the air out. If its air stuck in the line it might be worth a try.