Surface Gunk

Most of my tanks are plumbed into various central systems where the overflows manage to skim the scum off the top of the tanks, and the only visible symptom is what is floating in the sump. However, there are a couple individual tanks that are kept off systems because of special requirements that don't perfectly match one of the central systems or even instances where the bioload in one of the systems causes the scum to develop too quickly in the sump and needs to be controlled.

Like most other hobbyists, I've had my rounds with trying to get it all with paper towels, skim it with a cup, or even hold the siphon just right while doing water changes so it sucks the gunk off the surface. All of those resemble work too much for my taste, so I decided to find an easier way. Since I always have fry swimming somewhere in my tanks, it also had to be fry safe. The answer is actually a lot simpler than I thought it was going to be.

I simply needed an overflow that went to nowhere, but trapped something that had to stay on the surface, and wouldn't suck fry through an impeller or filter. Amazing how simple it looks when you see it on paper.

Since I knew that I could use an overflow to suck it off the surface, that's where I started. A simple piece of PVC works just fine.

Next is to power it, and is just as simple. I'm trying to suck water through an overflow without using a filter cartridge or powered impeller as the cartridge would prevent the overflow and an impeller would chop up my fry. Neither of those is an option.

What I ended up with at that point was a PVC "U" with a reduced side using a 1 inch airstone in the reduced portion to suck water down through the larger diameter section. This didn't quite match the bill as there just wasn't enough pull to keep the skimmate from drifting back into the tank. As a result, I decided to use a longer section of the overflow pipe and cut a groove in the side of it.

The groove is approximately 1/2 inch wide and extends 2-3 inches below the intended water line. In order to limit the amount of water traveling into the overflow below the surface, where the scum is, I cut another section of the same size pipe as the overflow, but wider to use as a gate to adjust where the flow comes from. The "gate" is held in place by cutting a couple notches in the edge that hold a nylon tie around it and the pipe. This allows me to raise and lower the gate to match the water line and adjust flow.

In order to hang on tanks, is simply a matter of extending the length of the pipes and using elbows and tees to hang it. It is important that the tees be set at the same distance from the bottom of the contraption so it hangs level in the water. Also, an additional tee needs to be placed on the outlet side relatively close to the waterline to allow water to flow as freely as possible. Too much back pressure from an improperly placed tee will not let the water overflow and cause a failure.

It has been about a year since I designed this contraption and put it to use. It has been my experience that the best cleaning takes place when the gate is approximately 1/2 inch below the waterline. This gives enough flow to the water entering to pull the scum off the surface as it enters the pipe without letting so much water in that the surface is unaffected.