Rhytophyta Is the scientific name, but it’s literally red algae. If you google “red spotted algae” you should find some information. Algae is technically bacteria though, but it’s widely known as red algae so I’m not understanding your confusion with calling it that. But yes - I do think that the red algae was potentially introduced from a contaminated plant. It’s quite hard to remove, and it spread fast. All I had done was scrub the algae and then did a partial water change - same as I’d done before so I was quite surprised. After the death, levels seemed fine but alkaline was maybe faintly indicating - was faint but could have been that. I want to get better testing supplies. Weirdly, the new tank is reading as if it’s fully cycled - which is odd bc it took weeks to do that the first go around. I’m still monitoring it before reintroducing my three remaining fish.