<a href=showthread.php?s=&postid=10618120#post10618120 target=_blank>Originally posted</a> by King-Kong
If I turn off one of the pumps on the 250, the water level becomes far too low to be of any use.
As it is now, I have to work hard to make it skim wet -- my valve is mostly closed; i'd say almost 80-90% shut off.
If I turn the pumps so they arent sitting on their skirted intakes then I get more water coming in, so perhaps if you turned the solo pump on it's side (isnt that how the 200 is), then practically closed your valve you'd be able to skim with one pump; I just have no idea why you'd want to.
Yeah, with the gate valve version (what is that, version 4 or something?) I can imagine the single pump soluion might present a problem... although you should be able to reduce the valve output enough in theory to raise it... maybe the ATI valve isnt adjustable enough as KK pointed out. I was thinking the standpipe versions would be easier though.
The point of skimming with just the one pump is then you would have less turbulence in that central 7" pipe (is that the right diameter of the central pipe on a BM250? Im guessing based on visuals). Throwing more and more air into the skimmer wont help if none of the bubbles can latch on to anything. It will just froth up the head so you end up skimming out more water. There is a combined 5000lph of air and water being shot up that central pipe... bubble plate or no bubble plate, thats more air per body area than any other skimmer on the market, except a beckett maybe... and its being done in a co-current manner. Thats not alot of time for much of anything to happen. Cutting the flow rate in half in this point... 900-1000lph in a 7" diameter, is at least back down to being in the range of what most other skimmers are. The max air that most other skimmer makers would put into a 7" diameter would be about 1100lph. I mean geez, 2000lph of air... that should have a 9-10" diameter pipe at least, and not be running in a co-current manner with 3000lph of water being shot up to the top (co-current doesnt provide the interface of counter current... the water that the bubble is with at the bottom of the skimmer is likely to be the same at the top of the skimmer).
So if making the neck taller is out, and raising the bioload is out... well...
The other thing you could do with one pump is have the port where the other pump attaches serve as an 'bleeder valve'. Wince the pumps shoot the water and air into a cyclone under the plate, the bubbles tend to concentrate in the center (lower pressure in the center, and air weighs less than water, so water is pressed to the outside by centrifugal force), so putting a valve where the other pump was should in theory only let water out, with maybe some of the bubbles. The benefit would be that you might be able to drop the turbulence in the skimmer even further because now you would have 900-1000lph of air, and rather than the full 1500lph of water, you might be able to get it down to 1000lph of water or lower. This would cut the flow up the central pipe to less than 1/2 of what it was before, and give the bubbles a chance to skim.
Thats about it... Im out of ideas on that one. Everything I can think of so far isnt permanent, so you can play around, and if it still doesnt work, maybe you can resell it and try something else. A BM250 is what... $750 new... I bet you could sell it to someone as is, make most of it back, and try out a different skimmer that is more suited to your tank size.
Heck... a 120g... yeah... no wonder the BM250 isnt working... you have 'overskimmed' in a way.
Or, you could do what I do... I have the skimmer on a timer so it only skims at night from 11pm to 8am. My skimmer is a less turbulent equal to the BM200 (DIY) on a 125g, and I think its still overkill. Im actually thinking a DAS EX-1 might be right on... maybe an EX-2 'just in case'... I can always turn off one pump and it will still run fine. Either that, or I put the 5' skimmer next to the tank and force feed it from the basement.