yes closing the return line would work via the gate valve. This could also do 1 of 2 things, slow flow through the sump, thus allowing micro bubbles more time to settle out, and decrease the likely hood of the cavitation. There for you would be effectively eliminating 2 possibilities. Which is not always the best if your looking to get to the route of the problem.
But on the upside, i think you have enough info at this point to figure it out. Let us know how it works out im interested myself.
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Two clownfish were in a tank. One says to the other, how do i drive this thing?
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