So my partner in crime did some more research:
"It is also helpful to introduce tannins with every water change. There are several ways to do this. The quickest way is to let a tea bag soak in the tank until the water gets light to medium brown in color. You can also brew the tea first, then let it cool to roughly the same temperature as your aquarium before adding it to the tank. Use a medicine dropper to add 10-20 drops of the tea to the tank. You can also add tannins to the tank by first microwaving a dried brown leaf for 30 seconds to kill any bacteria, then placing it into the tank." --http://www.cbsbettas.org/doc/articles/Tips_on_Spawning_Bettas.htm
We did this last night with a Roobios tea, as another site suggested, aaaand today I checked the tank and EVERYTHING is covered in a thick clear algae-slime... Ammonia levels didn't change... No idea if the these happenings are related, but holy yuck. The fish seem to be fine though.