You may try piling all the small rocks in a rear corner to act as a mini fuge.
Here is how I add sand:

Purchase dry, sugar fine sand. Soak in water change water anywhere from 24 hrs to a week. You now have the bio-film on the sand grains just like the prepackaged live sand available for purchase.

Turn off all pumps and powerheads.

Place sand in a zip lock bag, cut the corner of the bag and let it escape slowly to avoid a large sandstorm.

Only cover 1/4 of the sand bed with 1 inch of sand, or 1/4 inch or the entire sand bed per day.

Turn on pumps slowly as the water clears.