The main water problem you face with turtles as pets is one of keeping it clean, which in reality means changing it often. Turtles are sloppy eaters. For every bit of food that goes into their mouth, one or more bits are apt to fall to the bottom. They also appear to spend much of the time generating excreta, both in solid and liquid form. In the wild they can simply swim to a cleaner part of the pond if they feel the need. In a tank they cannot, and as we have been taught, any animal will eventually die if forced to live in its own waste. If you have a very large tank you may not have to change the water on a daily basis, but you will have to change it fairly regularly. You don't necessarily have to change all the water out each time. One scheme would be to change out half of the water every 3 or 4 days, and then do a complete change once every two weeks. Another thing you can do is invest in a filtration system. That will help keep the water clean, but you'll still have to change it, though not quite as often. A teaspoon of salt added for each gallon of water will help keep bacteria growth down and won't harm the turtle.
If your water is safe for tropical fish or goldfish, it is certainly safe for a turtle. Some people when first getting turtles as pets, will place a small fish or two, like a guppy, in the tank a few days earlier. If the guppy can handle the water, a turtle can also. If the guppy dies, the turtle may still do well, but you'd better take a close look at the water you're using, although most tap water should be OK.
Assuming the water is fine, you have suitable room for your turtle to roam, including land area, you are ready to bring home the turtle. Before introducing it to the aquarium, let’s take one more look at the decorations. First of all, when the water gets dirty, the decorations will as well, somewhat depending upon their texture and complexity. You may well have to clean the decorations when its time to change the water, and you'd no doubt like to keep this activity as simple as possible. No one wants to spend a Saturday morning cleaning turtle tank decorations!
A second recommendation is not to put anything in the tank that you've picked up from outside, such as a neat looking stick, without sterilizing it first. And finally, to safely keep turtles as pets, the decorations should not have edges that could injure the turtle, openings the turtle could get stuck in, or parts that the turtle might try to eat. Turtles will try to eat most anything, including plastic, so bear that in mind. Turtles as pets will even try to eat artificial plants, placed there as decorations, so you'll want to make certain there is nothing about the plant that can be nibbled off. After awhile the turtle will leave it alone. As far as small things to eat, this applies to gravel as well. Gravel is fine to have in the tank (though it must be cleaned too!) but if you go that route, get gravel that is at least a couple of centimeters (or an inch) in diameter. Only the very largest turtle might choose a piece of gravel that size to sample.
We mentioned earlier to put the turtle tank in a location where temperature extremes will be avoided. Give the little fellow some light as well. Not blinding light 24 hours a day, but decent light during the day, and low level light or darkness at night, and by all means keep the tank out of direct sunlight. Now you should be ready to get your turtle, and pick up a supply of commercial turtle food at the same time. Your job now is to monitor (and enjoy) the turtle's activities from time to time during the first few days to ensure that all is going well.
Make an effort to "visit" your pet at least once a day, if not several times. Most pets are acutely aware of being neglected, and many are more sociable than we might think. When you bring home a pet of any kind, you need to make a commitment to be a good steward, and provide proper care for the animal throughout its life (and turtles can have long ones), or until you find another good home for it. Turtles as pets are no exception here, even though they won't show the same emotions that dogs and cats will.