Put simply, you need a bigger drain line. If you think you don't, you are fooling yourself. I fell under the retail fantasy that a 1 inch drain line can flow 600 gph and supposedly more.
This is false
I read somewhere, which the link will be provided later, that a 1 inch drain line can safely handle 330 gph with NO bends or attachments. and that a 1.5 inch drain line cna handle 750 gph.
EDIT: link www.wetwebmedia.com/SystemPIX/.../Oneinchart.htm
I've found this statement is true. There is surging at any rate of 300 gph on my tanks with just a strainer and one 90 degree at the end before the drip tray, this was made worse at rates over 450 gph where there was a horrible sucking noise even with a silencing /vent tube.
Durso's helped but surging was still problematic and there was still a sucking noise that I could hear.
Wet/Dry section was completely sealed and with air constantly being forced into that section, duck tape was forced loose to allow venting. This wasted co2 with the added benefit of AWESOME oxygen levels.
With a 1.5 inch drain line the Wet/Dry section is sealed but no longer builds pressure and vents out through the drain line. Fresh air is still being sucked down with the water but at a much slower rate and when pressure does build slightly its forced back up through the drain tube and out the drain vent.
To ensure that it is indeed safer, I tested by dumping five gallons of water in rapidly. there was almost no difference in noise, and the overflow box was barely taxed. the drain line did not even fill up. Just smooth draining.
To also support my theory, a herbie method is quite by having a full siphon, AND a trickle of water down a larger drain pipe. (by larger i mean less flow for that dia. pipe is rated for) this produces no gurgles or air noises, and is relatively quite, the same thing is happening here just with larger pipes and larger flow rates.
picture of drip pan with that has been bored to accept 1.5 inch bulkhead. and new overflow box fitted with durso standpipe.
Durso was chosen to adjust height of water in overflow box, especially since the front weir has a 1 inch drop, and to make the drain as quiet as possible since there was no vent tube for this model
* The drain line is not dirty, its actually that color. the return line is dirty