SPS issues... again.

[ScubaSteve]

Ok, this is just my opinion here, so take it with a grain of salt...

ULNS keep corals "trim and fit" to keep those nice colours, meaning they have little "fat" (aka excess zooxanthellae) to help them ride out system disturbances. With ULNS, we are essentially keeping them on the knife's edge. This is why so many people in the beginning of biopellets were having recession problems: the corals were starving.

The dramatic change in lighting conditions (not just intensity, but spectrum) probably caused the SPS to freak out a little while adjusting. You didn't spike the lighting, so it's not shock, but being at the intensity you are now they might not have enough zooxanthellae or pigmentation to handle the increased photons rates. This can cause photoinhibition (meaning too much light is causing them to shut down) or they're heating up at the skeletons due to the lack of skin pigmentation (ie. coral sunscreen). This can (and does) happen when you swap to new bulbs, new fixtures, etc. The change in intensity from T5 to LED is probably what triggered it. Corals that aren't pushed to the limit by ULNS can usually ride this out without ill effect.

Also, a shift in spectrum (LEDs are narrow spectrum compared to halides and T5) can cause a shift in which zooxanthellae clade is dominant in the coral. Each clade has it's own specific absorption spectrum; if you switch to a different lighting spectrum, you can have a shift in clades as one is no longer being favoured over the other. This transition period might cause issues as there is a change over between the dominant algaes (ie. dominant food source).

If you are seeing basal STN, that signals to me starvation which makes me think the above. Try turning down the lights a little bit and feed a bit more. Try amino acids if you haven't already; I have found them to be particularly good at fighting STN.