Well, the zoos I'd posted about finally died. I had a small NH4 spike and it finished them off, along with one of my emeralds. I'm pretty sure the closing up of the zoos predated the NH4 spike, as I tested a couple days in a row after I noticed they weren't opening and got zeros. A few days later, I found the NH4 was up to .25. A day or so later the remaining emerald was having himself a nice dead-polyp buffet, so out they came. I am pretty confident that the NH4 spike was due to overfeeding, a loose xenia frag going through the return before I could get to it, and a dead turbo rotting away in the sump, so I'm still scratching my head about what initially caused the zoos to close.