Aquariums: Rescaped the 240 gal aquarium I manage, it went from clear to orange overnight…. where did I go wrong?

I manage a 240 gal, freshwater aquarium, usually stocked with native lake species (i.e. bluegill, sunfish, etc.) at a Visitor Center. I recently rescaped it with sand capped soil. Bottom layer is the pea gravel that was in the tank originally, then ~2 inches of potting soil, then ~2 inches of pool sand. It looked great! Then it turned orange….. I did several iterations of water changes and still orange. Please share some wisdom…
Where did I go wrong? I'm pregnant and soon my belly won't allow me to lean into this giant tank so I need to get it fixed asap! Thanks in advance