I think it's the kind of rocks you are using. You should try to go to Petsmart or Petco and buy the rocks there. Gravel….. i don't think would be the best idea to put in. But then again i never put in gravel.

Go buy diffrent rocks right away.

or empty the rocks out of the tank.

my husband and me went to a school yard and got a whole bucket of gravel rinsed it off and we have never bought store
rocks ever again.