If he looks healthy then he may just be trying to bask. Have you taken a temperature reading under the basking spot? For a healthy
Red Earred Slider (which is probably what he is) the basking spot temps should be in the 80-85 degree range. It is very important to actually monitor this temperature range. Otherwise, it is harder for your turtle to digest and they will sit and sun more often then they would have to otherwise.
I wouldn't worry too much as long as he continues to be alert and otherwise active. Like Picolio said, sometimes they lose their appetite for a few days. If it goes on for more than a week or two I would start to worry about illness more seriously. Turtles can go a long time without food but most aquatic species are ready eaters. If he continues this way you would then want to consider a possible upper respiratory infection (common in aquatics) or a heavy parasite load. If your turtle has an overload of parasites (common term for a variety of microbes) that could also cause a loss of appetite but it isn't commonly a problem for aquatics -- more for land dwellers.
You are probably going to want to look into buying a bigger setup for him ... like others have said, he will get a lot bigger. I would suggest going as big as you can comfortably afford since you don't want to have to keep upgrading every 6 months anyway. If you are really strapped for cash you can just use a heavy duty rubbermaid container ... it doesn't look pretty but your little guys won't really care and will be much happier in the larger setup. Keep in mind that eventually you are going to either need a 100 gal. tank or greater -- either that or an outdoor pond.
It is important that you make provisions for the UVB. They can't get UVB rays through the window, as Picolio said. You can buy a UVB florescent or coil from a good pet store ... just make sure you actually check to make sure it says UVB. You'll know by the price
... they don't cost less than $15 .. see if you can find an amount on the packaging ... you should go for at least 5% UVB ... most brands make 3%, 5%, and 8% tubes.
UVB is extremely important ... as Rav said, for them to metabolize their vitamin D3. Without it, they will suffer from something known as Metabolic Bone Disease ... or soft-shell disease. Their bones and shell won't be able to form properly ... and this can be fatal. If you can't afford a UVB light you need to make arrangements for them to recieve natural sunlight every day for at least a half hour. A half hour a day is thought to be about equivalent to a day's worth of UVB lighting. Just make sure that they are supervised the entire time ... turtles smaller than 6 inches are preyed upon by birds, cats, dogs, ants, and children
to name a few things. And do not leave them in the sun in a glass enclosure ... they will overheat. Make sure their area has partial shade as well.
As far as diet goes .. turtle pellets should make up about 25% of the diet because they are high in protein. You can probably get away with a higher percentage now when they are young though because young RES (and other aquatics) are highly carnivorous. I would add a lot of cooked meat to their diet. You can used canned tuna...chicken ... shrimp ... and crickets, earthworms, feeder fish, and snails are appreciated as well. You can get the insects from your local petstore ... either live or in cans. I don't recommend collecting them from the wild because you risk introducing parasites and pesticides. As far as veggies go ... try dark, leafy greens. If you can get them to eat orange-colored things like sweet potato and carrots that is good too -- high in vitamin A. They will appreciate veggies more when they get older.
I would also try a calcium supplement ... either cuttlebone (commonly sold for birds) or those turtle-shaped calcium blocks work fine. Don't be surprised if they bite off large chunks at a time.
Well, sorry for all the info. Hope I haven't overwhelmed you. But if I mention one thing I have to mention it all, you know?