Update time:
Well, after the mini-crash over the festive season, caused mainly by overheating, I spent 8 hours on a ladder with a scrubbing brush and removed the derbesia manually. After complete change of all globes- T5s and MH and a switch to 14000K MH's, agressive carbon and phosphate remover application and careful water parameter monitoring, I have managed to keep the system hair algae and fatality free for the last 4 months.
As a reward, I have decided to add some more livestock, starting with some elegance corals. I had a complete luck the other day when I popped my head into the LFS and found an enormous, fully expanded and healthy looking elegance that had been reserved for a display tank at a local casino. As they were late in collecting it- it had been in the LFS tank for 10 days, I begged to be allowed to take it... and won!
Attached below are some pics of it in its temperary positioning. I still want to place it deeper on the substrate in the next day of so. The pics do not do its size justice, it must be a foot in diameter- probably close to the size of a football!