If you provide a proper habitat for the turtle, the cats won't be able to get to it. There would be no reason you couldn't keep both pets. However, there is more to keeping turtles than most of us realize, so I'll give you a summary. After reading it, you can decide if you actually want a turtle.
It's recommended that a baby slider have at least a 15 gallon tank. Set it up so there's a land area and a water area. You want the water to be at least twice as deep as the turtle is long. If the turtle is two inches long, you'll want four inches of water.
In order to have healthy skin and shells, turtles need certain types of lighting and need to be warm. Air and water that are not warm enough can lead to infections. Turtles must have a basking area where they can get out of the water, dry off, and bask in very warm light. The ambient air temperature in the tank should be around 75 degrees, with the basking area warmer still. Over the basking area there should be some sort of lamp that will take a 40-60 watt incandescent bulb (or you can buy a ceramic light fixture made just for reptiles). If you live in an area that has farm stores, you can buy a metal light fixture made to keep baby chicks warm for just a few dollars. Don't buy the accompanying bulb, however. You need an ordinary incandescent bulb in the basking light. The basking area should be kept at 85-90 degrees. Use a thermometer to be sure. You can adjust the temperature by raising or lowering the light fixture. Since your tank is small, set up the basking area at one end - you don't want the entire tank to become too warm. The lights that come with the covers on aquariums are not suitable for turtles. You'll also need a submersible aquarium heater that will keep the water 78-82 degrees.
It's extremely important that you buy an additional light that produces UVB rays. A Reptisun 10. is a good brand that does. If you choose another brand be absolutely certain it provides UVB rays. Don't take the word of pet store personnel, but read it for yourself. Full-spectrum is not the same thing. I'm putting a lot of emphasis on this because it's crucial to your turtles' health. Without this light, Metabolic Bone Disease (MBD) will develop because your turtles won't be able to produce vitamin D. Vitamin supplements are not a good replacement for the proper lighting. MBD causes a very slow and painful death. UVA/UVB bulbs must be replaced every six months as they lose their effectiveness after that, even though they may still look fine. Light that comes through a window isn't sufficient because the glass filters out most of the rays turtles need to stay healthy.
Turtles are very sensitive to water quality. Even if you change the water every day, it can still contain harmful chemicals. A good filtration system is essential. Water changes are also needed even with a filter.
Feeding is another area where pet stores often give out bad information. Commercial food should make up 1/4 of the diet. Animal products (meat, earthworms, canned cat food) should make up another 1/4. The remaining half should be plant foods (dark lettuce like romaine, bits of strawberry or melon, etc.)
You may want to look at these sites for additional information on care and feeding.
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