Depends on your color preference. Some cheap options that work great would be pool filter sand, or play sand you can get at any home improvement store if you like whiter sand. I use black diamond blasting sand. It's only 8 bucks for 50 pounds. It's safe for aquariums, there's a few people here that use it. Just make sure you rinse it out really good before you use it. Some bags are worse than others as far as dust and difficulty of cleaning. Tool me roughly a half hour to do 50 lbs. pool filter and play sand on the other hand will come a lot cleaner and doesn't require as much rinsing. And that stuff is pretty cheap too. Under 10 bucks I think. And as far as changing I did mine all at once when I switched. I never had any ammo or nitrite spikes.