well first of all.. the turtle should definitely be in a larger tank... the 'rule of thumb' is 10 gallons of water per inch of turtle which means that your turtle should AT LEAST be in a 30 gallon tank.
but more importantly is the fact that it 'sounds' to me like your turtle has a Respiratory Infection. the best thing you can do right now is dry dock the turtle in a warm area for a few hours... then place him back into the water (making sure the water is warmer than normaly (probably around the mid 80's)... the higher temps help the turtles immune system kick in, and will also make the turtle more comfortable.
but definitely first thing in the morning you should take your turtle to the vet because the vet can give it some medicine to help it. you may also want to take a 'poop' sample (if the turtle has pooped) and have the vet check that for additional diseases and other things that turtles can have.
i hope this helps 
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}