The following caresheet can be taken as good base of information for the care of the commonly available sliders, maps and cooters.
Turtles or Terrapins?
In the UK we would generally class any freshwater dwelling chelonian as a terrapin. In America all bar one species is classed as a turtle. As the animals I'm refering to in this document originate from the US I will throughout refer to them as turtles.
Housing
The most commonly used enclosure for turtles is a standard fish tank. As a guide, when choosing a tank, ensure it can hold a minimum of 40 litres of water per an inch of shell length for the largest turtle, plus 20 Litres per an inch for each additional turtle. Bear in mind that this is water amount and not just tank size, and unless you construct an above tank basking area you will not be able to fill tanks to the brim. For hatchlings it's a good idea to plan for plenty of growth. This will not only allow for adequate swimming space but help maintain water quality.
No substrate is needed, but a single layer of large smooth pebbles or slate can be used. Avoid gravel as it can be swallowed and become stuck in the turtles digestive system, and sand because it can cause problems with filters. All but the very smallest specimens are great swimmers and thrive in deep water as long as there are plenty of places to rest just below the surface with their head above water. Water temperature for hatchlings should be 25-27C, turtles over 2” 22-24C, and standard room temperature (around 20C) being fine for older juveniles and adults.
It is essential to provide a dry basking area that allows them to completely leave the water, absorb UVB rays, and warm up in temperatures of 30-35C for 10-14 hours a day. A floating turtle dock, or large piece of cork bark that can be secured in place are ideal to create this area. A standard household spot bulb can be used as the basking lamp, plus a UVB bulb or tube with a minimum output of 5.0% (ideally 8.0 or 10.0%) must also be situated above the basking site. For larger tanks Mercury Vapour Bulbs (MVB) are great, as at correct distance they emit both suitable heat and strong UVB levels. Even if they are being kept without a male, adult females should additionally have access to a sandy area for egg laying.
Filtration
An often overlooked aspect of turtle care is the need for a huge amount of filtration. Internal filters just don't have the power or hold the amount of bio-media needed to cope with turtle waste. Without such filtration and the recommended amount of water, your animals are basically swimming in their own urine and harmful bacteria. An external filter rated for atleast 2-3x the amount of water is required. Personally I've found the eheim classic 2217 to be the best choice in terms of efficiency and price. For bio-media I suggest purchasing some alfagrog or lava rock. I would also recommend using water testing kits to monitor ammonia and nitrate levels.
Feeding
A good quality pellet (like ReptoMin) is best for making up the base of a turtles diet. Feed as much as would fit into their head if it were hollow once a day until about 6 months old, then slowly reducing until you are feeding pellets just twice a week by 4 or 5 years old. Greens such as kale, watercress, aquatic plants and dandelion leaves (ensure they have nor come into contact with harmful chemicals) should be constantly available. These should be placed in the water and left for the turtles to graze on until the plants are destroyed, or greens are soggy. Treats such as freshwater shrimp, small fish, or mealworms (fed on nutrient rich food) can be offered once every other week instead of pellets. For extra calcium cuttlebone should have the hard backing removed and be broken into more manageable pieces before being placed in the water.
Handling
Handling is not recommended unless absolutely necessary, as this will likely cause stress and potentially transfer dangerous bacteria such as salmonella. Always wash your hands thoroughly with anti-bacterial soap or use sanitizing gel after coming into contact with your pet or anything in its enclosure.
Tank Mates?
Turtles are by nature solitary animals, and don't have the desire to make friends with anyone or anything. Often fish and pretty much anything small enough will be considered a food item. In large enclosures it is usually possible to house turtles requiring the same habitat together. However there is always a chance that turtles will fight, or males will pester females, leading for the need to separte.
How Big Do they Get?
(Shell Length)
Cooters - males upto: 10" females upto: 14"
Sliders - males upto: 8" females upto: 12"
Maps - males upto: 6" females upt0: 11"