So... added three apple snails (and I think I have a mixed pair in there because two of them have been doing an awful lot of "wrestling") and 4 otos to combat the diatoms.
I drip acclimated the otos for 2 hours... I found 1 dead, 1 is still alive and seems well, and two are MIA. Are they likely dead? is it common for oto's to be MIA like this for days?
On the diatom front, the snails have nearly cleaned all the gravel. Hopefully they will get work on the plants soon.