I've finally got some aquatic plants established in my tank that are growing
and able to survive the goldfish. My goldfish are still under 3 to 4 inches
but they love to rip up tender plants.

I have pea gravel in the tank. The fish are getting bigger now and I've
read a lot of warnings here about the fish swallowing gravel.

My question is, what if anything should I put in the bottom of the tank that
will work for the fish and the plants in the aquarium?

Could I dump a load of marbles on top of the gravel?

Thanks.