I will try to present here a basic environment setup for water turtles. But keep in mind that this is a general indoor setup for water turtles. The best thing you should do is to find out exactly what kind of turtle you have because turtles' needs vary from one species to another. The more you know about your turtle the healthier it will be.
First of all any water turtle tank needs two basic areas: the swim area and the land area. Let's see what are the minimum requirements for both of them.
You MUST have a water level deep enough for the turtle to swim in peace. Therefore the minimum requirement is the water to be at least as deep as the turtle's shell. If you keep you water turtle indoor i recommend you to heat the water. The temperature is different for specific species.
The land area is an area that is above the water level. It must be well built so the turtle can climb on it when she leaves the water. The land portion should be heated with a lamp for basking purpose. Aquatic turtles don't need more then a rock ora a piece of wood but semi-aquatic turtles need some more space. They need the space so they can move more and choose different temperatures.
Minimum indoor setup for a small water turtle:
The best thing for your turtle, even for you, is not to use any substrate at all. First of all, the glass bottom of the aquarium is much easier to clean. Then you'll avoid having problems like your turtle swallowing some things it shouldn't. In many cases turtles eat rocks or sand from the bottom of the tank and you can deal with some unpleasant complications. Your turtles might become constipated or even choke on a small rock.
As i said before the land area it's a very important piece in the indoor turtle tank setup. You must be sure that your pet can easily climb on the land area. I suggest you should make it from bricks or wood because this type of material is very easy to clean and gets dry really fast. The land area usually has to be dry.
You shouldn't use too many decorations in your tank because the turtle won't have enough space to swim. But you can try adding some plants. My advice to you is to use plastic plants because they are as beautiful as the real ones and they make the aquarium look real nice. The problem with real plants is that the turtles will eat them and you'll have to buy new ones every week. So it might become really expensive. Plants are very good for your turtle because they make them feel like in the wilderness.
If you decide not to buy a water filter for your turtle tank then you should change the water every week or even more often if you think that the water is dirty. Try cleaning you tank as well as you can because because clean water is one of the most important factors in keeping a healthy turtle.
If you decide to get a water filter i can tell you that you won't be disappointed. I know that filters can be very expensive but they compensate by provoding you with water quality and work reduction