Need help from the overflow guru's

Im a freshwater kinda guy, i know i know..
Anyways, sumps just don't make it will out there on my side of the fence, but i have a CPR CS90 setup on my 75 gallon, a CS50 on my 29
both have 1 inch drains, cs 50 is quite and comfortable to listen to
the 75 gallon, surges and gurgles, i've tried durso and i still get surging though the gurgle is gone and much quiter.

I am under the impression there is a better way to solve this than to just apply a balm to the situation. like fish tanks, there is probably an underlying problem and im not finding it.
I've researched a little and i've seen a few general statements saying i need a bigger drain line. anythin gravity siphon with air should have a 1.5 inch drain for up to 750 gph??

assuming this is the case. is the cs 90 box able to be modified to accept a 1.5 inch bulkhead or can i leave that and just use adapters and have a bigger drain line

p.s. i use cpr wet/dry's for ease of sealing so i don't lose gobs and gobs of co2. this is a planted tank. throughput is around 500-600 gph

 

How about a gate valve between your overflow and your sump to dial in the flow.

 

Other way around, control the flow from your return pump. The water will only go out as fast as the return goes.

Welcome to 3reef HD blazingwolf. Nice to see someone else from TPT here.

 

Gate valve, or good quality ball valve on return as 2n10 said. may have to add a second ball valve just above the return pump that dumps back into the sump so that you're not holding the pump back too much.