Go to Home Depot and buy natural sea sponges. They are in the paint aisle.

Rinse the sponge out completely, spray it with a disinfectant like chlorhexadine. Let it sit for a bit and rinse it out again.

Wet it but don't squeeze out the water. Just let it drip out and pop it in a water bowl. Put the sponge/bowl in the tank. Place a piece of cardboard on top of the tank screen.

Repeat every few days. Leave more water in the sponge during sheds.

Good luck.