What kind of sand?(Post a picture)
Some sand is very fine(white play sand) and pool sand is coarser.
Denitrification is usually heavier in sand capped soils, so expect significantly more/larger gas bubbles when substrate is disturbed. It might become more toxic (hydrogen sulfide gases) also with less pwrhead circulation.
I'd personally experiment with a 2:1 cap/soil ratio to hasten cycling and clearer water for fish/plant introduction.