I think any type of sand or gravel works with aros as they cruise the top and rarely touch the bottom. Change the tank as in water change? If i remember correctly you have a couple aros with a few bottom dwellers and such, you might want to do 2 or 3 a week instead of once a week. Cleaning the gravel would be just tossing the gravel. As you stirr it up, all the waste and dirty stuff float in the water while the gravel sinks back down, and you just do a regular water change. OR you can get a gravel cleaner, it works with sand too. If you get the gravel cleaner, you could do it every waterchange, and if you decide to stir up the gravel, maybe once a week is good. Id recommend the gravel cleaner though as silvers are pretty jumpy and im not sure they're going to like your hand in their tank stirring up everything.