Updated
A pond-like smell in your tanks is typically caused by anaerobic bacteria. Add 2 to 4 tablespoons of store-bought 3% hydrogen peroxide and remove any old rockwool or dead plant matter.
The water in the tanks has a funky, pond-like, or swampy odor.
The smell is typically caused by anaerobic bacteria, which thrive in low-oxygen environments. This can happen if the water sits stagnant for a few days (e.g., the pump is off) or if old plant debris is decomposing in the tanks.