Can't keep SPS alive

[Aquattro]

You've probably mentioned this before, but what have you got for rock/sand? Nitrate should be 0 and PO4 as close to 0 as possible. Having enough rock and/or sand should eliminate nitrates all together. How old are the bulbs? Shouldn't matter, but good to know.
Digitata is known to grow like a weed, so 4 months and you should be pruning the excess.
What is the temp? And as Jack said, circulation?