Equipment:
- 5 gallon bucket
- garden hose + water
- sand to be washed
- small hand shovel (optional) -or other stirring device
If it's the 50lb bags of sand, then fill the bucket with about 1/4 to 1/3 of the sand. Turn on the water to the garden hose and start filling the bucket. Stir and mix the sand in the water as it fills up the bucket. When the bucket gets nearly full, tilt it so the water pours out of the bucket. You want this to be fast enough to not allow the silt to settle back to the bottom, but slow enough to keep the sand in. This isn't hard to do, and a brisk, controlled pour is sufficient. As the bucket is being dumped out you can either drop the hose off to the side or continue to aim it at the bottom of the bucket with one hand as you pour. You may have problems with finer sands if you decide to keep the hose aimed into the bucket, but I had no problems with the sand I chose to use. I used Quickcrete Medium Sand (comes in concrete mix looking thick paper bags.
Anyways I filled, dumped, and refilled the bucket a total of three times each portion of sand. You can repeat this process as much as desired (and depending on which sand you use), but repeat until the water you pour out is clear to your liking. Dump this clean sand into another container for holding or directly into the aquarium if you like. I suggest dumping in the sand first and then adding the water. To add water just lay a plate or bowl of some sort on the sand and pour water into the plate/bowl. This will stop the sand from getting disturbed during filling. I have also added sand after the tank was full of water (and even with a fish in it) with little problems with my particular sand. Some sands I hear of are very dirty/dusty and cloud the water pretty badly, so trial and error or playing it safe are your choices.
Enjoy and I hope this helps.