I think i will go with 6 leds and use one of those 24v commercial units. The cheapo LM317 constant current drivers are pretty lossy IMO
. I am happy to see that the commerical units combine a constant current driver (that can be dimmed) and a power supply all in one package for $33.00. An LM317 would come in handy for providing a 0-10v signal to the driver for dimming though.
So, 6 leds then. Should that be good for most coral requirements? (Over a 2.5g). If it works well, I will scale up for my 20g.