Considering your new test results, I think you could wait a little bit to see if the phos-zorb is going to help put an end to this problem. You may also be able to stop or at least slow the progress of your current diatom population by keeping the lights off for a few days. Otherwise, the only thing I can think of is exploring the idea of using a diatom filter. I'm not really sure if UV would help or not because all of my experience with them and what I have read/heard deals with their use on a SW tank.