RES (even hatchlings) are PERFECT swimmers and enjoy deep water. You should fill your tank all the way (without giving them a way of escape, common sense will have to do here) and provide a basking area with two lights (a UVB and UVA light) both aiming at the basking area.
Gravel is not good because turtles dont know better and end up swallowing the gravel which may cause impaction, which leads to expensive vet bills and possible death. A better substrate is a few river rocks, or sand (though it's a pain in the butt and you can end up killing your filter if you dont know what you are doing) is fine, but what I have had success with is a bare bottom tank, meaning no rocks, sand, nothing..it's easier to clean and pick up debri/poop and all other things, my turtles dont mind it and the water stays a hell of a lot cleaner.
You will have to use a water heater to maintain a water temperature of 75 - 80 degrees and have a good filter (i'd reccomend a 30 gallon filter for your tank) and purchase a thermometer to check the water and basking temperatue (the basking temp should be around 88 - 90 degrees)
However, this 15 gallon setup isnt going to last very long. I'd give it a few months until you have to get something bigger. RES, in the end require atleast a 100 gallon tank. For the next upgrade, go as big as you can
myrtletheturtle, you should look at this

BTW it would be good if you were to take that picture out of your post since new people who skim through posts might get the idea that this is a correct setup, which is not but I am sure you are working on fixing that

Be sure to read as much as you can and dont be afraid to ask questions. We are here to help..
