I decided to let them acclimate a bit before trying to fertilize, I didn't want to shock them more than they already were after being shipped to a different continent. Hopefully bottled water is sufficient, as my tap water is not the best.
Remember that fertilizer is the LEAST important of the cultural factors for successful orchid growing. Orchids need Very little. And for Pleurothallids in general, that is even more the case. They want pure water (little in the way of solids, even nutrient solids). So whatever you use on the rest of of your orchids, for this group use half or 1/4 as much. They can go a very long time without any! Too little, they may grow a bit more slowly. Too much, they're dead. So err on the side of "too little".
Well, whatever it's considered, mine HATED it and dropped 2 new growing leaves and one mature leaf. I'm down to a single leaf that seems to be holding on after an emergency media change. I'm crossing my fingers that it recovers, and sticking with pure RO water from now on.