*You will have to position the skimmer so that the inlet pipe is well below water level, to minimize disruption of the foam generated.

*Above drawing not to scale, and used for diagramatic purposes or as pathetic eye candy.

 

 

 I feel a little stupid posting what must have been done a hundred thousand times before by more talented DIY aquarists. Keep in mind that the specs are for MY small system. Pipe sizes will vary according to YOUR system's flowrates..

In-Line, In-Sump, Foam Fractionator
Materials:
1 (2?) mineral water bottles (acetate plastic) with cap
1/2" diameter PVC pipe
1/2" diameter PVC elbow
1/2" diameter clear plastic hose
Safety knife
PVC cement
Hot glue or Silicone or maybe even Superglue
Wooden Airblock(s)
Air Pump (not shown, sue me)
and one of those tiny hoseclips that comes with a suction cup.

Carefully cut a 2" long section of pipe
Carefully cut a 5" long section of pipe
With PVC glue, fit both into the PVC elbow.
Let the PVC assembly cure in a well ventilated area

Carefully cut off the bottom of the first bottle.
Cut a roughly 1/2" round hole in the side of the bottle (tough going?)
Insert the shorter leg of the PVC assembly into the hole
Do NOT glue. Just make sure the fit is tight. Servicing is facilitated that way.
If the force of the overflow is strong, use rubberbands to secure the bottle

Cut (or melt) a 1/3" round hole in the first bottlecap
Insert the clear plastic hose
Fill in gaps with hot glue or silicone from under the cap
Screw the cap onto the first bottle

Punch a small hole at the bottom edge of the first bottle
Insert the (suction cup) hose clamp from inside
Lock the wooden airblock(s) and airhose into the clamp

Cut the second bottle at midheight
Fit, glue, or do whatever it takes to mount the second bottle onto the longer leg of the PVC assembly. This creates a funnel to receive overflow from above. The funnel makes reconciling pipe sizes easier, and dismantling a breeze. Or you can plumb the thing in directly to the overflow pipe from above, and to heck with the funnel...

Finally I prefer to use cable ties to secure the inlet pipe to SOMETHING in the sump. I've tried using the suction cup that comes with the hoseclip, but it can be unreliable. Of course, if you plumbed the inlet direct to the tank overflow pipe, you shouldn't be reading this last part. I will readily admit its efficiency may pale in comparison to commercial models, but I have used a skimmer of this design (inside a plastic bucket sump) for the past 7 years, and it hasn't disappointed me or the corals I propagate. HTH

For questions or complaints, I happily refer you to Larry, to whom sole culpabilityfor the idea of posting this devolves Hahaha.