It started on the surface of a clay pot and spread over onto the heater glass and elsewhere like rashes. These red circular rings are smooth to touch and cannot be rubbed away easily. Are they inorganic or biological in nature. What are they.
You may be correct. From my research, there is only one species that lives in freshwater. While it is desirable in reef tank, not in my freshwater tank as the stains are out of place and hard to remove.Coraline algae? I've seen them in reef tanks but not freshwater.
It is an encrusting Red Algae (Hildenbrandia) <"Large Dark Red Spots">, but I don't think it is calcified.Coraline algae?
I think it is an encrusting Red Algae (Rhodophyta), but not the coralline one <"Pneophyllum cetinaensis - Wikipedia">, below.If it is indeed Coraline algae,
According to Wikepedia, rubra is a marine species. Its the other freshwater Hildenbrandia species. It said that it can tolerate severe desiccation. I just returned a stained flower pots after months of drying and curious to l see if the algae will revive.I have many on my rocks in my 15 year old tank. Noticed some spots under my sand substrate too.
Researching more it is called Hildenbrandia Rubra.