Bruce,
It was about 12 lbs. per gpm but that was for trout and flowing water.
As far as draining the pond if you have power you can drain it by using a regular 20 gpm sump pump from the hardware store. You can also get it started and then unplug it and as long as you have the outgoing hose on a downslope it will continue to run. Not as fast but it will continue. It would only take 15 hours to drain an 18,000 gallon pond with a 20 gpm sump pump. Actually a little more as the head increases but you'd probably have it drained in 24 hours.
I drain my 100,000 gpm ponds with sump pumps and even got a 50 gpm sump pump this year to do it a little faster.
Personally I think one diffuser would be enough in a pond this size. That's all I do and I have no problems. I wouldn't see the need for a commercial suface aerator if your densities aren't too high.
As far as the liner could it be your problem is you are trying to fix the liner in postion before the pond completely fills? If you let the liner slide down and compress by the weight of the water as it fills shouldn't that tighten it up good? Or do you get water coming up under your liner like I do in my well pump lined tank? This is only a problem for me in the winter when my well water is shut off.
My ponds although not lined have birms build up around them to keep most watershed water out which keeps them pretty clear.
I would strongly strongly suggest using the digout soil to even up the banks if you are digging into a slope. Steep slopes are a pain in the ### to keep the erosion down and get grass growing.
