Could someone suggest the best substrate for my turtle tank? I'm planning to use medium sized gravel so turtles can't swallow them.
Turtles like to swallow anything they can fit in their mouths. Most of the time you don't need to use a substrate for turtles. However if you want to use substrate for burrowing purposes or just pure aesthetic I would recommend using river rocks.
Example shown Below:
Using these types of rocks may be a little more expensive but turtles cannot swallow them and still encourages natural digging behavior along with a pleasing aesthetic.