I rinsed sand for 80 gal tank in twenty gallon rubbermaid tub.
Dumped about a quarter of the bag in the tub and while hose was running into the tub,,I sifted it with my hand's and then dumped out the water and repeated until the water I was dumping out, began to look clear instead of dirty.(took a couple hours to rinse about 100 lbs.)
I then dumped the clean sand in the aquarium,and placed another bunch in the rinse tub to clean.
no way in hell I would wash a cup full or two at a time, for such a large tank.
Would NOT rinse sand inside in bathtub,or sink, for it would/could very easily result in clogged plumbing,traps, under sink.
Keep filter intakes three or four inches off the bottom of the tank,and turn off filter's when aquascaping,or disturbing the sand so that sand does not find it's way into impeller assembly of your filter's.