If you have a HOB overflow box...
And the overflow box has a 1" drain, it can handle 600gph.
If your return pump is pushing more than the overflow box can drain, the display tank will overflow.
I do agree that you have way too many restrictions on the plumbing going back up to the tank. It should be one clean line going all the way up, without a ballvalve. I would suggest SpaFlex tubing.
What return pump are you using?
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