Hey Adam,
It's not that you need to lower the collection cup. You need to raise the water level.
To do this you will need to do a few things(not all just some). Especially since you are putting it in a sump and not hanging it.
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<LI>Increase pump size. You said RVT. is it a 800 or 600? You may need to go bigger than a MJ1200 even...
<LI>Put a ball valve on the output so you can adjust the water flow OUT of the skimkmer. This raises the water level within the skimmer.
<LI>Add a second airstone being driven by an independant air pump to increase foam production.
<LI>Move the input from the RVT up higher. This will raise the water level.
<LI>If anything I would make the neck longer. 6 to 8 inches. Use this thinner area for the actual foam production. Have your water level within this neck(five or six inches below the bottom of the collection cup). By having the foam have to go past a change in diameter a lot of the bubbles will break and you lose the pressure of all the bubbles pushing the ones on top up and out of the skimmer.
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The two I would do for sure is make the neck longer, much longer and add a ball valve. This is important as it lets you control the water level in the skimmer. Without this you are at the mercy of your pump and how fast the water leaves the skimmer for the water level. The others will help get a higher water level(pump) and increase bubbles(drier foam due to lower water level needed to get foam spilling over the lip of the neck.
Do that and your skimmer should work better.
As well make sure that at the spots where you get a change in diameter of the tubing, the edges are very very smooth and rounded. Sharp edges will cause premature breakage of bubbles.
[ 24 December 2001: Message edited by: DJ88 ]