Sump return pump too slow, then too fast

[ScubaSkeets]

Hi folks!
Just finished plumbing a sump on my 54G. I'm using a siphon overflow box for drain and a DC pump for return. Got it going, but I'm having a problem adjusting flow. If I'm using one setting on the pump, the water creeps up in the DT. If I slow the pump down by one number, the water creeps down in the DT. Help! How do I get it to be steady?

 

[wilsonline]

pics and video?
how is this siphon overflow?
This overflow may be the issue. Do you have a valve in the pipe that goes down?

 

[fishybizzness]

Hi. Do you have a gate valve on your overflow line? It's very essential to have for fine tuning the water flow.

 

[ScubaSkeets]

Thanks. Here is a pic:

But I think I got it going correctly. I had to very finely adjust the height of the overflow box and so far so good. I did not install a valve, but I'm thinking I am going to.