Some have mentioned the difficulty in sterilizing large volumes of water. While I'm not sure of the filters available for the aquaculture industry, I use some filters they were throwing out at my work (biotech). You can sterilize large volumes of water very quickly by 0.2 micron filtration. And that's true sterilization, for just keeping algae or cyanobacterial spores out of your cultures you could probably use a much larger port size and have the filter last longer, or filter faster.
At one point many years ago I set up a continuous culture of micro algae in an automated fashion that would feed my also automated rotifer culture. Old water from my main SPS reef tank would be pumped though a filter, and drip into a 5 gallon sparged (bubbled) glass bottle with my microalgae culture that would continuously spill (in such a way that the rotifers could not make their way back into the algae) over into the rotifer culture. All I really need to do was adjust the flow rate and size of the two bottles. Once that's right all you need to do is dump water into the salt water reservoir once in a while and the whole system ran itself.
