I got some buckets of sand from the beach! i used the same method Burgerking described.
I filled the tank with water first and then used an ice cream container. half filled the container with sand, put the container in the tank letting it fill up with water at the top of the tank ensuring no sand spilled out. let the container drop to to the bottom and emptied it. this way i didnt get any water clouding :-)

ive had sand before and its a big mess. If you get to close the syphon will suck it in