Wierd return pump/overflow interaction

[MSU Fan]

Background: bought the set up second hand - 50g bowfront which I added a sump/return/overflow set up.

 

So here's what is going on: when i feed my tank, I tend to turn off the powerheads and the return pump to make it easy to see and feed everyone, but also to keep the food from going into the overflow before the tank eats.  I've been doing that for many months.  In the last month or so, i would do the same procedure: shut down the powerheads and pumps.  water drops in display until it's below overflow/return siphon break.  feed the tank.  start the powerheads.  start the return pump.  Here's where it gets funny.  what seems to happen is one of a few things:

 

1. I lose the siphon in the overflow pipe.  both sides of the tube started below water line and I never saw them come out.  The return pump chamber runs dry, the ATO goes crazy.  I restart the siphon and it goes for a minute or so and then it just gets more and more air in the tube until the siphon breaks.  Or it will just work but the DT water level will be higher than normal.  

2.   I don't lose the siphon in the overflow pipe.  However, the water still builds in the tank until the return pump runs dry.  the water in the overflow chamber in the tank goes up right along with tank.  It's so strange because the overflow siphon still is full of water.  but water isn't going through.

 

I cannot find any breaks or whatnot that have compromised the overflow siphon tube.  I can't find any.  everything is working fine, I've cleaned all the parts and there are no obstructions.  I just can't seem to figure this all out.  Any help would be appreciated.

[M. Tournesol]

Could you give us photos or a video?
Hard to help you when we don't even know your tank brand.

[MSU Fan]

Here are some shots of the items in question. 

[M. Tournesol]

On 2/10/2022 at 9:31 PM, MSU Fan said:

1. I lose the siphon in the overflow pipe.  both sides of the tube started below water line and I never saw them come out.  The return pump chamber runs dry, the ATO goes crazy.  I restart the siphon and it goes for a minute or so and then it just gets more and more air in the tube until the siphon breaks.  Or it will just work but the DT water level will be higher than normal.  

2.   I don't lose the siphon in the overflow pipe.  However, the water still builds in the tank until the return pump runs dry.  the water in the overflow chamber in the tank goes up right along with tank.  It's so strange because the overflow siphon still is full of water.  but water isn't going through.

At first glance I would say, do a test with more water in the tank or reduce your return flow.

When you say "water isn't going through", do you mean no water go down your return pipe?

[PeterU]

On 2/10/2022 at 1:31 PM, MSU Fan said:

It's so strange because the overflow siphon still is full of water.  but water isn't going through.

Is your overflow tube too close to the bottom of the overflow box? Is there a snail or something in your overflow box?

 

With mine I found the tube length isn't equal on both sides, I put the shorter side in the overflow box. I also put something under the middle to raise it up so that I can have a higher display tank level.

[MSU Fan]

Ok,

1) No, there are no snails in the overflow box.  

2) the tube is equal

3) when i turned the pump off, the air siphoned in from the outer box as the display level rose.