i ran into the stn plaque about 2-3 years ago, i was losing colonies after colonies. some were over 3-4 yrs old... most were grown from small frags. it was a 300g 4+years old established and stable system.
the weirdest thing was that it wasn't all at the same time it seemed to be a couple at a time. i did have redbugs which were dealt with with no subsequent problems. but that wasn't it. every pests were checked for, different brand test kits were used, all rodi filters replaced, fresh salt water tested, holding containers replaced, every equipment broken down and maintained, fragging, glueing, puttying, dipping, feeding more, feeding less, increased flow, lessened flow, more light, less light, increased temp., decreased temp, lotion potion, voodoo....nothing helped. all these over a period of almost a year.
then i accidentally nuked the tank with kalk reactor about 14 months ago. lost 99% of the sps and over 90% of other livestock. it was devastating but you know what? after 3-4 weeks of massive water changes(80g of salt water that i have on hand at all time plus cleared the shelves of a couple of lfs's 18 boxes of natural sea water the first day alone, then went through 4 boxes of 200g esv).....literally starting over except with the same rocks, i now have a thriving colonies of acro's grown from tiny frags again.
it's been a blessing in disguise???? i still don't know what the problem was. if it were bacteria/viral/pest, etc, i guess the kalk took care of that. i had no more stn problem....cause i had no acro left....:duh:
i hate to say it but you may have to "start over"....perhaps begin with massive water changes.
gl, i hope things get resolved....one way or another.