Backflow issue

I am setting up a new 55g tank. My return line is a mag7 that goes up through the tank to a loc-line output.

An issue I noticed during testing is if the power for the mag7 goes out, it the locline creates a suction in the DT and starts to bring water back down the the sump.

The only way I can figure out to prevent this is to keep the output line pointed slightly up, so as the water line drops, there is a point that it catches air and then stops the backflow. However, with this solution and I limited to only 1 spot with the output line.

Is there any other way to correct this?

 

Blackflow valve

 

This or you have to drill a couple small holes to create a siphon break up close to the surface. A picture would help but you could do it at the fitting before your locline on both sides. It has to be high enough so your sump won't overflow.

 

AZdesertrat says it best and this is the rule that I went by when setting up my system. You don't want to rely on anything that could potentially fail, ball valve, check valve or drilled holes.
Check out AZdesertrat's post in the link below.

http://www.3reef.com/forums/filters-pumps-etc/how-do-you-prevent-backflow-84464-2.html

 

Link didn't work but quoted his post

 

That's what I did, I raised the output device from the pump out of the sump high enough to the surface that in loss of power it won't overflow the sump. I tested also to make sure it wouldn't overflow the sump when back flow happens.

 

As long as you keep the return close to the surface so it breaks suction after a small, very easily calculated amount of water flows back so it is exposed to atmosphere and you maintain sufficient freeboard in the sump you will never have a flood related to the return. No maintaince, no parts to replace and nothing to clean, just a simple air gap.