A check valve above ground is illegal in Texas, Michigan, and other states that understand a negative pressure in an underground water line can lead to contamination issues. The main reason I don't like them is the water hammer that is caused by any additional check valve other than the one on the pump.
Looks good, but you need to wrap that galvanized nipple with some good electric tape. Screwing steel into brass causes electrolysis and will eat that nipple out in a few months if you don't wrap it..
Better off with a brass nipple, dissimilar metals don't get along.
I haven't seen a contamination issue that can be directly linked to a check valve at the tank, although I agree there is a possibility of it happening. Usually air will be pulled past the Oring in the pitless.
This is awkward, but...
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