your sump is way too full as well…hopefully this is a power out pic and not an operating pic, other wise you are going to have flood troubles with back siphon….
if your fish store told you to put that valve and check in the drain line i would seriously be looking for abetter store, as they know very little of what they are talking about..
you should remove the plumbing from the drain, an redo it wit a simple drop as mentioned and add either an air line or open top…
you really should either buy or build an overflow box to control and skim the water properly…
you should also redrill the hole for the return to also accommodate a 1" bulkhead, then either bush it down to 3/4" or use a valve to control the flow better. or somehow figure out how to make it an emergency drain, and then just run the return up the back side of the tank and over the top.