From my other post (which is directly below yours

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"As much water as you can put in without your turtle climbing out the top (turtles are very crafty and strong, they can extend their front legs pretty well and have enough muscle in them to hoist their whole body up over the tank). If you have more than one turtle in the same tank, turtles have also been known to climb on top of one another and climb out. For one two inch turtle, I'd say filling it up and leaving 3-4 inches is fine. For two or more, you make your best judgement.
Oh, and it's a good idea to check if your stand (or floor for apartments or new houses) can withstand the weight of the tank and water. Glass is pretty heavy, and one gallon = roughly 8.33 pounds of water, so expect your final weight to exceed 100 pounds, and as much as 600+ (If you have a large tank).
Red-Eared Sliders are aquatic turtles, they spend most of their time in and under the water, they eat, sleep, and live in the water, and only enter land to bask or lay eggs, so you don't need to worry much about them not knowing how to swim and drown, even hatchlings have the basic instinct to swim and dive. Only exceptions are say if the filter is too strong and sucks the turtle in and it can't escape (generally not a problem for normal turtles bigger than 3 inches), or if they get lodged in a tight space and can't get out (the plastic black bars on a Zoo-Med Turtle Dock for example)."
A common solution to all of this, is to simply make/place a cover of some sort over the tank, so the turtles can't excape. A good one would be a screen cover, but don't buy the ones you see at pet stores, the mesh on them is too fine and the holes are too small that they can filter UVB rays out of your UVB light source in your basking area (if you have one set up). I heard from someone that at least some UVB is filtered out no matter how big your screen holes are (unless they're huge, which wouldn't prevent a turtle from escaping), but 1/2 inch holes are a good minimum.
Hey hey hey.