I'm coming back and forth regularly on that topic. I want to use them because they are far less bulky and distracting than an independent skimmer. But each time I try, I have to give up because they are just not very efficient.
First, it is a trade-off between surface and bottom aspiration, and for skimming to be efficient you have to give up a lot of bottom sucking. Which means you lose a lot of the laminar flow that is essential for good CO2 distribution.
Second, it is very difficult to tune up precisely, and the tuning changes a lot with water level.
Last, you can't put a shrimp guard on it, otherwise the bottom sucking is so low that the skimming part gets too much flow and wobbles or get sucked down.
Last, I suspect the physics of it cause a lot of flow rate loss, because the flow in my tank is so much better without it.
In the end, my current position is to use an independent skimmer, or better yet (what I'm doing now) none at all, but with a powerful enough canister filter that the output Lily pipe is doing enough surface skimming on its own. I got rid of any skimmer two weeks ago and so far things seems to be better or similar (except I have some leaves debris on the surface, but I can get rid of that easily). To be fair though, a lot of experienced aquascapers like Dennis Wong "don't run a tank without" a skimmer. But they do so to be able to inject a
lot of CO2 in their tank, which I'm not good enough to handle properly.
Hope this helps
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