I use Tunze skimmers in my AIO tanks, although I’m not sure which model, if any fits in a bio cube.
I will say that none of the compact AIO skimmers work as well as full size “in sump“ models. You may just be spending money to switch from one not so great skimmer to another.
What is the issue with the Icecap? Most of these types of skimmers can be dialed in eventually. They also almost require an ATO because all can be finicky about water levels. If you dont have an ATO, I would recommend spending your money on that first, then give the Icecap Skimmer another try.
If you are having something going on not related to performance, like it’s too noisy or having mechanical issues, I would recommend you reach out to BRS and see if Tunze makes a model that fits. I have a 9001 in my NUVO 20. It’s quiet and has been running well for over a year after some work to dial it in properly.