Do not use gravel in a turtle tank. They will eat it and it can cause serious problemlems - even death. If you do want to use gravel it must be very large gravel so that they cannot eat it but you may also just have a bare bottom - it is easier to clean also.
You can use a coffee filter in first than put your water in than the gravel and that is how you separate gravel and water...:) or you can get a screen like a drainer and pour all of your gravel and water in that.....:)
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Other than using gravel for building, gravel is useful to farm for flint. When you mine gravel with a shovel, there is a small chance that instead of dropping a gravel block, it will drop flint. Flint can be used in crafting recipes such as flint and steel as well as arrows
Stains are popular, but don't offer the same range of color and protection. You'd want to use a good primer and use an acrylic based paint on top of it. Most places that sell paint usually mark cans as "outdoor" if they're sufficient.