PLEASE HELP! - Durso Standpipe

[samrowe1]

I have just done a water test to see if my plumbing is sealed, and it is. I decided to test my standpipe and return pumps but now have a problem.

I filled the tank with a hose and the water went down into the sump, I waited until the water in the sump was a few inches above the baffles as I knew the return pumps would lower the level slightly. I then stopped the hose and turned on the return pumps. They began to work and filled the tank higher than its previous set level. The tank water level kept getting higher and the level in return chamber lowered until there was no water left for the pumps to pump.

My obvious thought was that the standpipe cannot accommodate 6000LPH but this is not the problem, only a small amount of water falls down the standpipe and I have no idea why.

As this is my first saltwater tank, I am always learning and I never knew that a weir was watertight until not long ago, I thought the only purpose was to stop fish getting into sump/hide plumbing. For this reason the display tank water level is at the same level as the rubber band in this picture I have linked: http://gyazo.com/cb2b9975c7f666c766b7badabff9238b

This could be a reason but I am not sure, I will be very grateful if anyone knows what the problem is. (I have attatched a photo of the durso in my tank + my aquascape just for you to see)

Thanks, Sam