My fish tank is too deep

I have a fish tank which has been built too deep: 90 cms. (x 150 cms x 60 cms)

I were not able to clean the bottom enough and I dismantled it after a couple of years, leaving it empty for now.

I am thinking to fill the tank with a bottom of about 40 cms. But with what, with gravel, bigger stones or sand.

Anybody who can give me an idea?

Here is it how it was before: http://www.antoniuniphotography.com/p470131130/h58da05eb#h58da05eb

Most of the Java Barb died as we had to leave the house for a couple of months because of the floods. The Three-Spot Gourami I rescued from the surrounding fields when they were drying-out after the floods.