I agree, a vet visit is in order.
Also....a few general health tips:
As large a tank as you can get...the minimum size for a RES is 10gallons of water for EACH inch of the turtles shell. For an adult RES you will need at least a 75-90gallon tank or equivilent sized contanior.
Lots of water...fill the tank up as high as is pratcial with water to maximize the swimming space.
Heat lamps...your turtle needs a basking spot outside of the water in the high 80's. Place the bulb on top of the mesh lid so he cannot get burned and track the temps with a digital thermometer.
UVB. A UVB bulb is EXTREMELY important. This is a bulb that gives off sun-like rays and aids in your turtles processing of calcium. Without a UVB bulb your turtle will not be able to turn calcium into bone and the results are a serious of frequently fatal disorders. YOu can buy UVB bulbs at any pet store that sells reptile supplies. Most people use the florescent strip lights and put them in a florsecent light fixture. Bulbs can be misleading because many regular heat bulbs will claim to give off "UV rays" but they are not talking about UVB. One way to tell if you have the right bulb is that they are quite expensive...at a petstore the bulb itself ususally runs about 20-30$
Diet. Feed your turtle a varied diet, with the staple being a good turtle pellet like Reptomin. You can also indlcude shrimp, either from a can at the pet store or fresh. Crickets, earthworms, meal worms, Superworms(when the are larger), brine shrimp, bloodworm, or beefheart frozen cubes (found in the frozen fish food section of your pet store). Also veggies should be incorperated into the diet as well including dark leafy greens such as collard, mustard and dandelion greens. It is trickier to get a turtle to eat veggies and you must be persistant but they usually do.
Cleaning. A good filter is imperative, as are freqent (in a tank as small as a ten-gallon probably weekly or biweekly) partial (30%) water changes are needed. Use a siphon, like the ones that are used for fish tanks, and siphon out 30% of the water replacing it with fresh water. How often you will have to clean depends on your husbandry, you can purchase some good (Sera is a great brand) testing kits and test for Nitrate and Ammonia, when these levels start rising it is time to clean the tank.
Most importantly get the turtle to the vet ASAP, injested materials are very dangerous and may require surgery to remedy. Turtles are quite time consuming and expensive pets, although they are rewarding if you love them enough. Its too bad someone got it for you as a present as they are not really good spontaneous pets (although no pet is!) if you really dont think you can provide proper care for your turtle throughout its life (and they live a looong time) then i would recomend finding someone educated about turtles who knows how to provide for them.
good luck!
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0.1 Bearded dragon
0.1 mexican kingsnake
1.0.2 Leopard Gecko's
0.0.1 Rose Hair Tarantula
1.0 BTS
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4.1 Cats