What I did after a lot of hit and miss was buy a bag of quilt batting and cut a large square that I loosely wrapped around my filter. I found that kept the pea gravel from clogging the impellers on the pump. Then I would just use pea gravel to fill the bucket to about 3 or 4 inches from the top. Then I would put some larger stones, maybe 2 inches at the most, on top of the pea gravel to keep the fish from mouthing around in the gravel looking for tidbits. I could tell when the filter needed flushed when the flow rate would slow down. The batting material would be nasty with silt. Just tossed it, flushed the gravel until clear then refill the bucket. I kept two buckets on hand. It made cleaning easier.