The 18-gallon illustrates an interesting phase in the progression from diatom to green algae growth. I added another layer of coral to the existing condo a few months ago. In the photo above, the older layer of coral is visible underneath as green and the newer layer above as reddish brown. I had assumed that the diatom stage occurred only once in a tank and that additional coral would simply evolve from white to green. But apparently the diatom stage is a necessary part of the green algae growth process for any new coral introduced into a tank.
Here’s a closer view of the difference between the two layers. I extended the condo to accommodate the additional small colonies from the tanks that I decommissioned earlier.
Here’s an even closer look at the opae’ula grazing on the algae and diatom. The new stratum of coral has already begun the process of switching from diatom to green algae.
The diatom, in this close-up, appears to be more orange than brown or red. The ghostly pink blur on the upper right of this photo is an opae’ula that has molted, shedding a smaller exoskeleton for a larger one to accommodate growth.