You do not want root tabs releasing anything into your water table. It can result in a terrible algae bloom and make the water turn to a color similar to pea soup.
I alway recommend against the use of root tabs in any aquarium because they can cause more problems than they correct. I have grown beautiful 'root feeders' such as all types of Crypts. and amazon swords and never had the need to use rood tabs.
If you feel the need to use them, put them (into the substrate only) where you can remember were they've been placed and then don't disturb them until they spend themselves. This is usually 3 months time.
Len