Sorry it took a while to get back but I think that algae turf scrubbers could work extremely well. They are already well used (of a fashion) in reef setups.
In a reef setup one would use an algae like Caluerpa (sp?) in order to suck up some of the nutrients (like phosphate and the like) that are far more problematic than in FW. This is often augmented by fertilising the refugium or sump with a mineral rich mud to encourage growth.
The Great Barrier Reef Aquarium Aquarium (in Townsville Australia) is a 4 million litre reef system where they are trialling turf algae. They have found that 1000 micron mesh allows for the best growth/cleaning and utilise a tipping bucket system to increase gas exchange and nutrient takeup in the ensuing turbulence.
They have found that shallow trays work better (somewhat expected due to the increased area for gas exchange).
An important point I have seen is the need for "grazing" the system from time to time (at Townsville they do this every 5 - 14 days) to prevent further organisms growing in the scrubber that may feed off the algae and start to dump these nutrients back into the water. This is done by removing the tray and scraping the algae off with a flat perspex sheet (so as not to remove all the algae).
It was found after the tests at Townsville that while most conventional methods (mechanical and biological) of filtration leave nutrient levels in the order of parts per million, algae scrubbers render them in the parts per billion.
As an added boost it is thought that all the harvested algae could be used in fuel generation of one sort or another.
My conclusion on this is that algae scrubbers appear to work fantastically, and assuming you can gain the right algae to seed a freshwater tank they should convert over pretty well. The big problems I have are somewhat obvious:
1 what effect will having such an effective nutrient remover and oxygenator have on a planted tank? The obvious answer is not good for the flora. However in a fish only system (particularly one with large messy predatory fish) it could be a fantastic thing. Especially if teemed up with a sump and fluidised sand bed filtration system.
2 How effective is the algae itself. Remember that in a reef system the liverock can complete the nitrogen cycle (ammonia to nitrite to nitrate to nitrogen gas) so there would not have been such a high concentration of nitrogen compounds. However if coupled as mentioned above it should perform admirably.
3 Is there a risk from algae going "sexual". In reefs the common used caleurpa sp again?) can become sexually active. I forget how but this can cause an entire tank to crash and wreak some amazing devastation.
4 What are the lighting requirements? Would it be a 24hr lighting (like reef fuges?)
My final thought is then that this is a truly exciting prospect if maybe a little unsuited to planted tanks. It could help reduce the need for water changes to a reef level (of 10% per month) instead of the frequency currently necessary in FW. Certainly something I will keep my eye out for in the future for more information on its implications for FW.
/www.reefs.org/library/talklog/m_lidster_100498.html
