I have no idea why the mag reduced the population so much. Perhaps there was something else in it too. It's just an anecdotal observation - not science.
Right now my mag is down to normal levels (1350 - mag eliminates Bryopsis at around 1700) and the pods don't react to any dosing.
I bet a Mandarin can plow through 20 dollars worth of pods in a day or two.
That's why it's so important to get them trained onto frozen food.
If you haven't already perhaps consider doing it now.
Think of it like you would if you were getting any other kind of pet.
You wouldn't get a cat relying on her being able to catch enough mice to sustain herself.

Then in addition to that get some cheato from somebody else's tank to get that life and feed your fish and perhaps add live phyto and it should grow from there.