OK, that makes sense. ABS plastic pipe is commonly used in drain systems in
homes, sinks,toilets and such. It's black. PVC is usually white, although
it's grey when used for electrical conduit. The ABS fittings used to plumb a
toilet are perfect for connecting the 4" pipe to a tank or in your case a
filter or skimmer. They're called toilet flanges and have holes in them
making it real easy to bolt to a surface, use silicone sealer to make it
water tight. To get an idea of this, look at
http://www.skippysstuff.com/biofiltr.htm and check out the links to his
pictures. Hope this helps,
Paul

"PoconoRobin" powlusr at ptd dot net wrote in message
...
I'm not familiar with ABS pipe, but will look into it. Both the bottom
drain and skimmer will be gravity flow into a 55 gallon settling tank. I
feel I need the 4" for the gravity flow.
--
Robin
The Poconos, PA