Invert, sorry for the long delay in response-- that doesn't usually happen. I let the post slip by, but after reading it today I wanted to let you know that I think it is possible for some tunicates and sponges to become acclimated to a tank (I got lucky in this setup). For the most part I have seen them decline over time in pico reefs, especially the vividly-colored ones such as those bright orange sponges or the purple tunicate colonies that turn up form time to time in LFS displays. I got lucky in that the understructure of a lr chip I purchased had a yellowish sponge (gen/sp?) and a transparent tunicate colony that seem to be faring well, if it started to rot I was just going to throw out the frag but instead it spread to the other lower rock surfaces, it's one of my favorite specimens and I'll get some recent pics on soon of it.
I do feel it may be possible to keep these tricky feeders in a technically-designed nano, but the hitches willl be maintaining sufficient plankton density and cleanliness along with chemical stability. These stressors should be less pronouced in larger systems and trickier to deal with in nano tanks.
I think an experimenter could build a one-way circulation system where water proceeds from a larger, stored reservoir through a pico and then into a collection bin- so the chemistry stays rather stable while there is always a constant influx of balanced water with dense planktonic loading. At this point, the system is not really a true pico! I feel it would not be likely to keep these planktonic feeders alive in sub-picos or pico reefs where one is trying to achieve a true closed ecosystem balance; the real rewards in this arena come from being able to keep stony corals and several other well-suited specimens alive with minimal maintenance hassle. Larger nanos are better suited to keeping sponges IMO