Mr. Wilson,
Way back in post #6164 you have a picture of a check valve at the end of a return line. Are there advantages to doing it this way as opposed to placing it in-line and just before the outlet to the tank?
Some people drill a siphon break hole in the return line just below the surface of the water. When the return pump shuts down, air enters the hole and breaks the siphon so the sump doesn't overflow. If you drill the hole at a downward angle it will become a venturi and suck air in. If you drill it pointing up water will be forced out of the hole, but it will eventually plug with debris, crystalize salts, and cover over with coraline algae.
A better system is to use a swing (flapper) check valve to draw in air to break the siphon when the return pump is off. Using a check valve to govern the direction of water inline is less effective as it can seize if calcified, or plugged with sand, algae or snails etc. If you use a check valve (wet) inline, it is forced open for extended periods. You cannot rely on the valve closing when it finally gets called to duty. On the other hand, a check valve that is forced closed is only partially exposed to corrosive salt water and detritus and critters cannot jam it closed. As soon as the pump is turned off, it will always open and let air in. This method also avoids any restriction of flow.
I use true union, clear check valves so I can see if there is any kind of blockage or wear to the rubber, and clean or replace them if there are any issues. Ball or spring style check valves are noisy, require strong pressure to open and can be unreliable. Swiss Wye style check valves are supposed to be good, but I have never used one. They need a certain amount of pressure to get a good vacuum seal and they need to be cleaned a few times a year.
The check valve should go as close to the end of the line as possible, and can be vertical or horizontal. Even though no water will ever come out of the end of the check valve, make sure it is directed over the tank or overflow box just to be safe.
One issue with back siphon is having enough space in the sump to accommodate the extra water. The other issue is exposing corals to air as the water level drops. If you don't plan well you may also end up with a system that splashes the lighting and spills water out of the tank when the pump comes back on and the water level is low.
Always perform a test power outage and simulated drain clog to make sure your fail safes kick in the way you think they will.