My son want a turtle tank

Tortoises are much easier to take care of than aquatic turtles. I'd recommend something like a Russian Tortoise over a turtle anyday. But if he really wants an aquatic turtle, you can set the tank up like you would an aquarium, but with as much filtration as possible and a substantial land area. You can't overfilter a turtle tank. Turtles need a basking lamp and UV lighting for proper growth. Most are more carnivorous when young and switch to a more herbivorous diet as they age.

I wouldn't recommend anything smaller than a 75G for the most common aquatic species (Red Eared/Yellow Bellied Sliders, Painteds, Maps, etc.) They all get fairly large and need plenty of room to swim and move around.

However, if you were to go the terrestrial route and get a tortoise instead, their care is quite a bit easier. A big wooden box is fine (as long as it's tall enough so they can't climb over), and they need heat and UV, but only need a water bowl big enough to soak in, and they are almost completely herbivorous.

Here's a couple good sites on turtles:
http://petturtle.com/
http://exoticpets.about.com/cs/reptilesturtles/a/aquaticturtles.htm

Get a soft shell turtle. They have known to live over 25 years. You can get baby size to palm size for $25-$50 dollars at pet shops. Not much needed for them thrive and live. Just like fish, it spends all it's life under water. Sometimes you'll catch the turtle stretch out his head above water and peeks around before it goes back down into the water. Very active swimmer, might even be friendly enough to recognize it's owner. You will need; Heater, Aquarium light, some sort of rock substrate for the flooring, floating pellets food, any type of filter will work and medication incase it catches bacteria on it's shell. I wouldn't recommend anything smaller than 75G either.

I would just go smooth pebble rocks or sand for the floor covering, this is so it's easier for the turtle and cleaning don't have to be so difficult. It will eat floating pellets most of the time, or give it a feeder fish here and there once in a full moon.

Look into them, they're 100% water turtle.

I have two Red Ear Sliders in a 500g pond right now. They're wonderful pets, but i wouldn't recommend them for young children unless there is DEFINITE parental supervision/assistance in tank maintenance and with caring for the turtle(s).

The posts above all provide great information, but i'll contribute what i know about aquatic turtles even though i'll probably be repeating most of what's already been said.

It is illegal to purchase aquatic turtles under 4" in shell length unless it is for scientific or educational purposes. This law was passed after a huge salmonella outbreak in the 1960's-1970's, mainly caused by infants putting these small turtles into their mouths/handling the turtles and not washing their hands thoroughly after doing so. However, it obviously isn't enforced very well, because baby turtles are still readily sold to the general public today.

Anyway... there's your short history lesson. :thumbsup: Now, here's what a proper aquatic turtle habitat should consist of:
1) 55g+ aquarium for 1 turtle. You can probably get away with having two in a 75g. Bigger=better!
2) UV light *AND* a heat lamp for basking. The UV light helps in synthesizing vitamins (especially D3), which is necessary for the turtles to metabolize calcium.
3) A good, quality filter. When i kept my turtles inside, i used Fluval in-tank filters (and they worked very well), but a canister filter is probably even better.
4) Basking area. Zoomed makes turtle basking docks that float and attach to the side of the aquarium with suction cups. These work well for juvenile/young adult turtles, but i'm not sure about heavier adults. You may want to look into that.
5) Decorations if desired... HOWEVER, it is a lot easier to clean turtle tanks if they are bare-bottom, especially since you will likely be needing to drain most/all of the water on a weekly basis. But if you do decide that you want some gravel at the bottom, don't use anything that is small enough for a turtle to consume, as i have frequently heard of them eating small pebbles (which can obviously be deadly). You could also put in some anacharis, but they love to snack on it!

Anyway, hope this information helps... they're great animals, but they are frequently put into small, dirty aquariums as babies and are given away when they become bigger. If provided the proper living conditions, they can be wonderful pets and i highly recommend having one. However, they typically don't like to be handled much, so make sure that your son realizes that before you make your decision.

Check out a red footed tortoise they're fun and friendly too. I hand feed lettuce and such to the ones at work.
I agree with whoever said aquatic turtles can be a heap of work unless, as AzFishkid said, you get a fluval internal filter or a canister, either would save hours of frustration changing nasty water, and a bare bottom tank is much, MUCH easier to maintain.
All turtles can live for quite a while, so make sure there is much time spent considering your options.
I went with a ball python (my avatar), they (and almost any other non aquatic reptile/amphibean) are much easier to care for and maintain and you can handle and hang out with them a lot more than a red ear. My BP becomes a necklace when its warm enough, bearded dragons and geckos are fun too tho so maybe show him a little bit of whatelse he could maybe choose from to get and he might like something else even more.