check valves...

Check valves are a fllod waiting to happen! Drilled holes are not far behind since both require cleaning and maintenance and there is still no guarante they will work when needed.
The only foolproof method of preventing backflow or backsiphonage is an air gap period. The best way to accomplish this is to keep the returns just slightly below the surface so in a power outage only a very small, easily calculated amount of water siphons down to the sump. Always maintain that amount of freeboard or spare room in the sump and you can sleep soundly at night and never have to worry about a flood.

My display is a 100G and my sump is a 30 G. The returns are 3/4" below the surface, about the same as the weir or fingers on my overflow box. I know that my tank holds 4.7 gallons per vertical inch of water (60"x18" divided by 231= 4.67) , (3/4" inch drop is only 3.5 gallons) so I always maintain that amount of freeboard in my sump at all times. I actually keep more like 8 gallons of room because the skimmer works best at that level.

Check valves get a grain of sand, alge, slime or food on the seat and fail, it does not have to be catastrophic, even a trickle will flood given time. Drilled holes get snails parked over them, some food or algeas stuck on them, calcium build up, salt build up etc and require regular cleaning and will still fail when the snail parks on it. Air gaps never go away and require no maintenance.

 

Keeping the air hole clear on the return is not a big issue. Every other week check it out with a toothpick and you will be fine. But as a safety precaution, have your return tubes as high as possible and not deep down in the tank.

Welcome to 3reef AZ. Are you still on RC?

 

No, they booted me from Team RC and from the site after 7 years. They are really protective of their sponsors and don't like to admit some are less than honest in their claims. Oh well, I have found better sites such as this one.

 

thanks az