My little one's can be quiet smelly to, but as long as you keep the tank clean eg. good filtration then you should have no worries.
My tank is inside with 2 urles in it, and it get's a little smelly sometmes ( if the house has been closed up ) but in saying that , you wouldnt notice it unless you were up close to it.
I am going to put my babies in an outside pit once they are bigger as I think the more they grow the more they eat poop and this is what makes the water smell.
Our turtle don't put off any odour, but we have good filters in with them. We have a large tank that has 2 filters, one internal filter and a big canister filter, without both filters, the water does get a bit smelly. However overfeeding can be a big cause for bad odour too.
i have 3 turtles together, and they have never smelt, we have a really big filter and pump that was pretty pricey, and it keeps the water crystal clear. unless you allow your turtle water to go nasty, they shouldnt smell at all.
Years ago we tried our Turtle on some pellet food, and after eating this it really fouled the water and stank, so I have never used any pellet food again.