The saga continues, @DaveKS: the tank went green, did a 72 hr blackout, completely changed the water down to the substrate last week, and everything seemed fine. The new inhabitants arrived today and when the lights came on, the water was noticeably chalky again. I'd pared the oak leaf down to the size of an IAL over the weekend, but it's been in there since 10/22.

Is this just a really slow cycle or some other sort of bacterial issue?

I added three alder cones today but assume it will take days before they'll have any effect.