It started with a removal, and a desire to do something to my angelfish tank, so I'd feel more motivated to care for it. I've gone lax with water changes- for a while they were happening about once every ten or twelve days (instread of the usual seven), then two weeks went by and BSA started to show up on glass and anubias leaves, and I felt guilty. Did a larger water change and rinsed one of the filter sponges. I hadn't tested nitrates before working on the tank but the day after they were below 20ppm, so that looks good.
Pulled out one of the fake plants. Snails have been scraping them- probably to eat the algae but also removing part of the surface and then little bits of white float around the aquarium (I think that's what they're from).