I just put the tunze nano on my tank 3 days ago. It has been producing a pretty wet skimate. It seems to really go nuts after I do a water change. It has larger bubbles than I am used to. I am hoping that after the break in period these will get smaller and I will have a darker foam.
I tried cheaper skimmers and couldn't get any skimmate at all, so this is already skimming. I have an extreemly high bioload also.
It fits nicely in the overflow in the back. Only the cup sticks out the top. The water level must stay accurate. If you have any evaporation you must add fresh to keep the water level up to where it needs to be for proper skimming.
Like I said, I just put it on. Maybe someone with longer term use will chime in, but in my opinion, so far, so good.