Well...
One in the 20H wasn't moving last nite, moved it to the plant tank where several were doing fine. Dead this morning tho.
Tested the 20H water:
ph 7.6
ammonia under .25 (barely registering)
nitrites ~.4 (I rarely see ANY nitrites, but this is a fairly new tank and i just added 10 more platy juvies 2 days ago. maybe i hit a mini cycle?)
nitrates ~15
phosphates 5
Doesnt seem that bad except for the nitrites. moved all 3 from the 20H to the 10G plant tank anyway. All the remaining brigs are in there now. plant tank is a 10G with ~6G in it. Did a PWC on that one today too, just to be sure. nitrates nada, phosphate 2. Gonna leave 'em all in there and check daily. Temp is 73, should be OK.
Yeesh...worst luck I've had in ages. Either they're way more sensitive than i expected or there's something 'else' in my water I don't know about.