Stubborn pest - mite or possibly aphid

[harpspiel]

I am just about at my wit's end here. My enclosure is slowly getting overrun with some kind of tiny pest, which I often see crawling all over the roots and leaves of my orchids - it seems to especially like Schoenorchis fragrans. It could be benign since I haven't seen much damage yet, but I am seeing a few brown pock marks on the bottoms of leaves. The critter is white or cream colored, about 1 mm long and when I've looked with a magnifying glass seems to have two antennae and around six legs, a little round body and no wings. My two best guesses are either some kind of aphid, or some sort of white spider mite. There are no webs, though, and the tank is around 90% humidity and gets down to 59F every night, so that's not supposed to be conditions that spider mites thrive in.

So far, I have tried spraying the plants and the whole tank with insecticidal soap for a few weeks, which seemed to have no effect.

Yesterday I tried the dry ice method, sealed off the tank and completely filled it for about 4 hours. That killed off all the fungus gnats which I didn't care about, but today the critters are out and about just as happy as they were before.

I don't know what to do from here or how to identify them. Do I try dry ice again, but for longer this time? Do I try to introduce some sort of beneficial predatory insect to control the population? Do I tear the whole tank apart and submerge all the plants and try to bake the soil? I have a shipment coming in next week from Andy's, and I can keep the plants in a short-term setup briefly, but I really need to fix the problem. Any suggestions at this point are appreciated!