Basically you can pick between the following two problems:
1) As already noted the pump is two big for the sump, solution keep pump throttled back or replace with smaller pump.
2) Sump isn't designed to handle the flow rate, solution redesign sump or reduce flow rate.
I would suggest that until you get a better handle on things and some more experience you simply keep the pump throttled back, there is nothing wrong with this. Very few people have wide open returns.
A picture of the sump and setup will allow for more interpretation of the issue.
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