I'm using a Tunze 9001 on mine, not sure a 9004 would fit ... I modded the 9001 to have the magnet in the side, rather than the back. Fits perfectly in the middle compartment. There's many YouTube videos of WB20s with a Tunze 9001. Can't go wrong with it.
if you are down for a little diy I used a dremel to shave the sides to fit into my water box 20 and this thing is a work horse probably a little to good at skimming but I pull outside air with it and my ph bottoms out at 8.2 and peaks at 8.5 without kalk or any other ph boosting products
Exactly what I came to post. I run one on my 25 lagoon. It's fantastic.
Be forewarned. Their water level sticker may not be an ever remotely close proximation of the correct water level.
I had to raise my skimmer about an inch higher than the sticker said. But for real, it's hands down the best nano skimmer I've ever used. And I've used a bunch. Icecap k1, air drive, hand blown glass air drive, tunze, and the IM. For far less money the mini q is far superior in almost every way.