| | Re: crushing tabs for sand?
I would not do that.
Pool filter sand does not compact, so no need to mix fertilizer in any attempt to spread it more equally. Roots will grow through it just fine, and find the fertilizer.
Water does flow through this sand, so any fertilizer mixed with the sand will end up in the water column. Fertilizer that is at the surface of the sand will almost immediately enter the water column, and the water will become a fertilizer-rich soup that encourages algae.
Crushing the tablet will allow the fertilizer to become available to enter the water column faster.
The most I would do might be to cut a tablet in half, and sink the half-tablet all the way to the bottom of the tank (touch the glass) so it is well buried.
Other options for fertilizing the tank with slow release is to use Osmocote in a 00 gel cap. The gel cap will take some time to melt, then the Osmocote is slow release, too.
In a low tech tank you do not want the fertilizer to get so rich in the water column. Leave the fertilizer in its tablet, and tiny traces of fertilizer will end up in the water column, at a rate much closer to what the plants can use.