Fading to yellow

Pink-tipped green anemone in winter colours.

Anthopleura elegantissima

Out on the shore, these anemones are green, with pink or very pink tips. In the tank, and especially over winter, they gradually lose the green algae that give them their colour. These algae need sunlight to grow. Indoors, there are only a few hours of filtered sunlight; in winter, in my apartment, they get only artificial light.

Most are olive to bright green (depending on the species of algal symbionts present) with tentacles tipped in pink. Individuals that live in microhabitats that are deficient in photosynthetically active radiation (PAR), such as under docks or in caves, lack symbionts and are pale yellow to white in color. (Wikipedia)

The anemones on our shores are a very bright green, unless they're hiding under deep rock overhangs.

Anemones, Edgewater beach, 2009

From a paper referenced in this Wikipedia post: " ... anemones display phototactic behavior and may  move to regions of the cave that produce the best physiological fit between host and symbiont species." (David Secord, 2005)

This may possibly hint at the reason all of the pink-tipped green anemones in my tank congregate along the front wall, where the light is stronger.

The internal algae, busily converting sunlight and carbon to carbohydrates, provide oxygen to the anemone hosts as a by-product. Without this, the anemones survive, but do not reproduce as quickly. However, in my tank, a year ago, there were 5 of these anemones. I counted this afternoon; now there are 19, and one is busy splitting in two, to make it 20.

Morning sunlight should be arriving at their window sometime in April. If it's strong enough (I may re-direct it with a mirror to be sure), some of the green colour may return.

Looking again at the first photo, I am reminded that a powdery green algae grows everywhere in the tank, on walls, shells, even on hermit crab carapaces. But not inside the anemones; it's obviously not the correct species. And it likes artificial light.