So, once upon a time, I had a sebae nem, now I have a BTA and a Carpet in my new tank. Nems are going to move until they find a happy spot, and if you turn your flow back on, and that spot is no longer happy for them, they will move again. Personally, I would turn on all flow and let it do its thing for a week, if it doesn't find a happy place then I would be concerned and possibly turn off the 12-watt lamp and wait another few days, if that doesn't work, turn off the 20-watt and turn back on the 12-watt and wait... there are bound to be areas in your tank with the right amount of flow, you just gotta give it time.