Can you use Planttabs tablets or Flourish Tabs with an under-gravel
setup?
Would it make the whole water environment too rich for the fish due to
the
circulation?

Thanks
I would just be careful to use some tabs with low first number in the N-P-K
system.. Most of the N in comerical ferts is made up of ammonia, which im
sure you know isn't that great for fish.
Ryan