If it's a commercially-made aquarium, just clean it, test for leaks, and proceed as usual.
If it's a custom or DIY tank, consider the fact that it may have been built with a lower water level in mind if designed specifically for turtles...meaning that the glass thickness or other construction details are not intended to withstand the increased pressure of filling it up all the way. I had a plywood turtle tank for many years that was made from leftover scraps of lumber and glass from other projects. It was 6 x 3 feet x 3 feet tall, but the glass was only 1/4-inch, i.e. way too thin to withstand the pressure of water 3 feet deep. Filling that up all the way to use as an aquarium would have been a disaster.