Whoa man, there is so much you need to get and do. I dont know where to start.
After a couple of month's (close to a year) your turtle should be around 3 to 4 inches long depending on how much you feed it and what you feed it. An overweight turtle is a unhealthy turtle and an underweight one is just as bad.
What exactly have you been feeding it? RES should have a varied diet of vegetable's, aquatic plants, insects/fish, and commercial food pellet's. Not just turtle food, turtle food alone doesnt cut it. Another reason why he may be small and not growing are parasite's, in that case he need's to go to a vet ASAP, especially since he is showing symptom's of a respiratory infection, which is FATAL if not treated by a vet.
You need a water heater to keep the water at a constant 78 degrees, and 82 degrees for a sick turtle. Without one your turtle can sick with a respiratory infection (what your turtle may have). Get one ASAP.
You also need a UVB light in addition to a heat lamp for proper bone growth and to prevent disease's like shell rot and etc, which is also fatal.
RES also need deep water, they love deep water. Keep in mind in the wild they swim in realy deep lake's.
You also need a water filter.
And you need about 10 gallon's per every inch of your turtle. 3 Turtle's will require a 340 gallon tank when they are older and that's if they get along. If not you will need a couple of 100 gallon tank's for each of them!
I realy realy suggest that you read ALL the info on here:
www.redearslider.com
and please take your turtle to a vet before it is too late and it die's. Keep him warm, and as stress free as possible. You have a long way to go..but I guarantee you will succeed if you realy try, turtle's are NOT easy pet's to keep and require alot of time and care and hundred's of dollar's spent.
Good luck, if you need any extra help feel free to PM me.
