That's interesting to see those African violets growing so nicely in there. If you might ever have time I would be interested to see the planters for a view of the root systems. Do they have a lot of roots?
Which plants have the scale on them? Do you know what kind of palm that is? If there are scale on the palm I would say you could just dunk the whole plant underwater (right in the tank if there is room) for about 12 hours and that will drown many of the scale. If you repeat that a few times you should be able to kill them off.
I imagine the African violets will be damaged if you put them underwater.
I got scale in a riparium once and I was able to eliminate them by just carefully inspecting every day and squashing every one that I found. It took a while, but eventually I got them all. An advantage of growing like this is that the insects can't hide down in the roots.