Most potting soils contain fertilizers which are great for plants, but
very harmful to fish. I would get rid of the potting soil altogether
and use a sand or gravel base, especially since you want to keep fish in
there too. Remember to use an aquatic plant fertilizer (read label to
make sure it is safe for fish) once a week and don't vacuum the gravel
around the plants to thoroughly.

Hope this helps.

Vicki