Completely closing valves downstream of a pump

Look up Cavitation and shutting down the water outlet on any pump especially a cheap aquarium model is not good for them

I would put a low water cut off on it .

 

The return in your display should only be slightly below the surface so a very small easily calculated amount of water backflows before the return is exposed to atmosphere and the siphon breaks. In my case my tank is a 23" high 100G and the sump is a 30G, my returns are 3/4" below the surface so I can potentially siphon just under 4 gallons back in a power outage. I always maintain 8 gallons worth of freeboard or extra capacity in my sump at all times, even when topping off so floods are not an issue.
Drilled holes and check valves both require maintenance and will fail eventually, it could be 10 minutes after cleaning, you never know. Air gap siphon breaks don't fail and require no maintenance.

 

I have tested it and my sump easily handles the extra water. I was more concerned about one of my small damsels getting sucked into it during a power outage, which would be a very unfortunate way to lose a fish.

Thanks for the info guys, that question has been bugging me for a while.

 

i understand what your saying, and i should have mentioned that the damage free zone is only short term

 

I was working in a large factory many yrs ago had a few large pumps we where installing and servicing some older ones and they ran from 100 to 175 HP motors and one of the valves down stream where cut off totally on a 100 hp pump well there is a formula that converts H.P to heat from cavation and then the housing becomes a steam boiler less and safety pressure limits that that pump exploded like a bomb killed one guy next to it I was about 100ft away and It knocked me over . But its never ever wise to run even a little tiny aquarium pump closed down . The steam can be made in a little bit and you can bust the impeller or crack the housing or blow out the chevrons .