They could be.
I have kept fish that like digging to various extents.
A fish that just picks up a little bit and spits it out is probably not going to find or dig up the tabs. They move around the tank trying a little here, a little there.
Fish that are more serious about digging could indeed find the tabs. When the tabs are exposed to the water they will release the fertilizer a lot faster. I am thinking about my Rams which kept digging holes where the fry could hide. Or this one Clown Loach that dug a hole all the way to the bottom of the tank.
Fish that tunnel under the substrate (Kuhlie Loaches, Dojo Loaches, Yoyo Loaches IME) are really bad news for tablets. They really stir up the substrate.
Smaller critters that do this (MTS) do not move very much substrate when they burrow through, so they are OK.
