Whoever sold you a 200# sand filter as a 70 GPM filter was deceiving you, either accidentally (because they were ignorant) or deliberately.

To get good results at 70GPM, you need at least a 30" filter -- 600# of sand. Even a 24" filter needs ~350# of sand.

To get definite answers, you need to post pump & filter maker and models, and send photos to poolforum AT gmail DOT com