I'm an API rep, and I'm very sorry to see that you've had a bad experience with the nexx. I'd like to address some of the issues put forth in this thread.
Correct me if I'm wrong, but it sounds like you're considering the round foam pads as a "prefilter." That's not the case - the cartridge (and its frame) is considered a self-sufficient filtration medium, but so is the foam pad. The nexx concept is something of a hybrid between HOB-style filtration, and that of a canister. The foams are an option for those hobbyists who like to mix and match media to the specific needs of their aquariums. The filter is not designed to use both both the foam and a cartridge in one chamber (Again, correct me if I'm wrong, but it sounds as though you had both the foam and the cartridge present in each of the chambers, which could definitely lead to a leak - there's not enough room for both). The idea behind using the foam is that it provides mechanical filtration, and then the user can add to the chamber a pouch or two of Nitra-Zorb, Phos-Zorb, Activated Carbon, or even loose media that have been sealed in a media bag. One of the best things about our most popular filter line, the Filstar series of canister filters, is their versatility, and we wanted the nexx to be every bit as versatile.
I'd also like to address the Bio-Chem Stars. As an experienced aquarist, I can definitely tell you - no joke. We've done mercury intrusion tests, and the amount of surface area present in one star provides space for a bed of bacteria sufficient for five gallons of (normally stocked) aquarium water. The pore size is ideal for the formation of colonies of nitrifying bacteria throughout each star. As far as trashing them peridically...well, I'm not on board with that either. Those instructions were written using ceramic noodles as a guideline, and the stars are a different animal. You won't get the same clogging issues you would with ceramic rings, and because of that, and the durability of the resin, there should be no need to replace them as frequently as stated in the instructions. Use your own judgment on that. Here in our test facility we have some that have been in continuous use for years.
I hope this clears the air a bit. There are bound to be some growing pains when a new piece of hardware hits the hobby, but we're listening, and I just wanted to let you all know we're here to help if you have any questions. The original set of instructions leave a bit to be desired - this seems to be the cause of most of the problems you had with your nexx, monty67. We've fixed that, and you'll find unambiguous written instructions at marsfishcare.com
Also, don't hesitate to use the toll-free Tech. Services help line, at (800) 847-0659. You'll usually get to talk to a tech directly, but if you go into voicemail, leave a contact number and your question - we'll always call back within one business day. Everyone at that number is familiar both with aquarium keeping AND the entire product line, so no question is off-limits.
Hope this helped!