I bought a Tunze 9004 (AC) a few weeks ago after bouncing back and forth on it due to reviews. I must say, this thing is awesome. They seem to have changed the design over the last couple of years. Particularly with the air adjustment mechanism.
I had it pretty dialed in after a couple of days, and it produces a skim that I have to empty every 3-4 days. This is in an IM Fusion 40.
I used the Tunze 9004 previously. It worked fine but I didn’t like fitting the skimmer body together after the full tear down for cleaning. It was quiet and reliable. I would buy it again - the DC model is interesting
I’ve also had a Tunze 9001- quiet but didn’t produce much. I probably would not buy that one again.
Before the 9004, I had a Warner Marine WS-1 (same exact thing as the Aquamaxx and the JNS version). I liked this skimmer. It was not as quiet as the Tunze but it was close. Produced a bit more than the Tunze. I would buy the Aquamaxx version now.
I have experience with the icecap k1 nano, worked suprisingly well but the issue with these tiny needle wheel pumps is they get clogged sooo easily. baby snails, macroalgae, debris, etc. If you clean your pumps on a monthly or bi-monthly basis its not too bad.
Have a reef octopus 1000s classic HOB. Seems to work pretty well after some adjustments and removing a foam block that just caused it to overflow. Using on a Oceanic biocube 29.