Osmocoat has been used by planted aquarist since the 1980's.
Lots of use in the past. It's slow release, it's fine to use about 5 grams per sqft or so for most applications.
Make sure you have it on the bottom of the glass, then cap with sediment. Last about 6-9 months for most aquarium temps. Dissolution rate is fairly constant(temp controlled).
You can also add more later as the aquarium ages(do not add except once every 6-12 months, more will not help), by removing a 1/4 section of the top layer sand and pouring it in, then covering again.
Some used clay(cat litter unscented etc), or clay like soil and add water, mix in the osmocoat and make mud ice cubes, these are inserted into the sediment where they melt and make a lot less mess this way.
Up to you.
Regards,
Tom Barr