Have you tested the powerfail with your skimmer running? The skimmer will hold some water that will dump back into the sump as well. As will any reactors that you might run (carbon, gfo, etc).
While that does appear to be working well in terms of flood resistance, I myself would lose sleep over trusting that those return nozzles stay at that level forever and always. Here's what Murphy could possibly do to you one day...
You are in the tank and decide to move the nozzles away temporarily so you point them down while you are working in the tank. Or maybe you bumped them down by accident. Or maybe I guest was playing around and moved your nozzle. You are done working in the tank and forget to return the nozzles back to their upright position. Power fails while you are away from the tank. FLOOD.
I suspect you will be keeping a close eye on those nozzles for a while but if they get moved while you are not home that could be disaster waiting to happen. Or you may be tired one day and forget to check on the tank to make sure those nozzles have not moved either by accident or on purpose.
At any rate, I would lose sleep over those nozzles :-) With a check valve you would give yourself a larger margin for error here and since you already have the check valve, why not install it?
Or if you really want peace of mind without the check valves, install all your equipment, point the nozzles down all the way (this being the position that they could accidentally be placed in even if you don't intend for them to run this way during normal operations) and do a power fail to see if your sump holds the water. If not, drop the level of your refugium enough so that your sump does hold all the water in the event of a power fail.
Just my two cents :-)
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