Welp... after quite a bit of research, I think the BBA I’ve been getting in the goldfish tank might be caused by my enormous water changes.
I found out that my light fixture is giving me low-light levels of PAR so that’s not it. The other main culprit that causes BBA is fluctuating CO2--I’ve got steady CO2 flow from my DIY, so I didn’t think that could be it either. However, I found out that tap water often contains high levels of dissolved CO2, which means that 50% water changes with tap, especially water changes done with a Python, will cause a huge spike of CO2 into the aquarium. Ta-da, instant algae.
I guess there are a few ways to remedy this. First and most impossible would be to let the water sit for a while in buckets before adding it--for 50% changes in a 46 gallon, that would be incredibly inconvenient. The other method would be to just do smaller and more frequent water changes, which I’m sure would be better for the fish anyway.
Sigh. Aquariums, I tell you. Well, time to douse everything in hydrogen peroxide, I guess.