Like greech said, it's a balance between the pump capacity and the overflow capacity. The only thing, though, is that the balance and the water level was OK until you placed that reverse durso.
A few things I suggest you check. Check the height of the drain tube in the internal overflow. Did you inadvertently push it up higher? Regardless, try pushing it down deeper.the durso in the overflow is at the same height. actually if i remove the cap water will enter thru the top of the Tfitting. Next, check the airvent in that drain pipe. If it is clogged, water will not flow efficiently in your drain. Get something that fits in the vent and goes it's full length, and see if it's clogged, then dribble some RODI water down the vent to make sure it is indeed clear.vent is clear as i am still running tap water as not to waste all that saltwater
If both the above check out OK, then what changed when you put that reverse durso in was that all those angles and additional tubing caused a restriction in drain flow. To correct this, replace the tubes you used with larger diameter tubing. Going from 3/4" to 1" will help, and if you can change all those fittings with larger diameter will also help. Like using a large 2" T like I posted in your other thread.
