Hi John.
Uhmmm, here's a theory of mine as to why many peltate and ramose corals experience base-tissue die-off once in a hobbyist (or nearly any)
marine aquarium.
The natural reef often has light hitting a coral from all sides
...owing to diffusion of light through the water column, (when I snorkel I look horizontally and my eyes receive blue light, and so should the coral). Add the superior nutrition a coral receives in the wild and the discrepancy between wild and captive conditions increases.
If you change the environment, the coral will always be affected.
Keep in mind that base tissue die-off happens in the wild too, seasonally even, and has certainly occurred in many a captive specimen of coral that nevertheless went on to grow. What you're experiencing may be offset by growth where tissue does receive adequate light.
The serious worry sets in when a well fed coral experiences tissue recession even in its well-lit portions.
hth
horge
PS: Looooong ago, I once dodged this effect when placing a peltate colony (
T. reniformis) low on the rock superstructure and next to the
sand substrate. It was still brightly lit from above, but now there was light bouncing off the sand (which had a formidable population of 'stirrers' and 'scavengers') as well, lighting the coral's underside.
There were likely other factors involved (what was technically my refugium amounted to 3x the volume of the display, and puking out plankton and nekton like the blazes, and the current was pretty snappy --crosscurrent turbulence), but the result was surprising. Two years later, the coral was given to a friend, and I heard later that its base-tissue did recede.
