How big is your turtle?
8cm or so, fits in my palm
How long have you had it?
3rd year now
What is the water temperature?
varies with weather, min 24C max 27C
Did you use a thermometer?
no, according to my room temp/country temp (singapore,asia)
Are you using a water heater?
no
How much water is in there?
unsure, 1/3 of a regular fish tank, around 90x40x60cm
Are you using a water conditioner?
no
Are you using any filtration?
yes
What is the basking temperature?
28-30c
Is there a basking light?
yes
Is there a basking platform that is easy to climb on?
yes, but this guy nver climbs..
What kind is it or what is it made out of?
diy, wood/plastic
Is there a UVB light?
yes
What have you been trying to feed it?
greens(very little, sometimes spits back out), bloodworms.. dried shrimps..cuttlefish bone..
When was the last time your turtle ate?
the night before i found it overturned
How big is the tank/pond/enclosure?
around 90x40x60cm
Is the tank near a window?
Is the tank in a room with a lot of activity?
Have you read the Basic Care section?
yes
Have you searched the forums for similar situations?
no, but i have looked through quite a lot
Is there any other unusual activity/symptoms?
lethargy, unable to swim, decrease in appetite, swollen flesh, soft shell -.-
yesterday:
As above, i had to go to college yesterday, my mum called me at 8am and said that she though he had died, she found him overturned, not moving.. she moved it and it sort of struggled and she left it alone.. came back at 1pm, he was overturned, i turned him and immediately he came up gasping, i could hear him gasp for air.. i knew something was wrong so i took him up and examined him, his neck was limp out straight, at the time i didnt know it was drowing, but i put him in a separate shallow water tank. It laid its neck out and head came up every few mins or so, opened mouth wide and gasped. When i picked it up, everything came out, head,limbs etc. Even its plastron was bloated, i could push it in abit and its area between front legs and neck would come out.. left it there for awhile and by nighttime it could lift its head up well and would hide when it saw me or touched..
Today(1day passed):
however, it isnt eating well yet, just gobbled 3 dried shrimps tonight, and stayed in the same spot forever, in the shallow tank. Will struggle when lifted. I noticed its plastron was very very soft, near the tail i could push and move it easily, and as mentioned, his flesh was swollen compared to the rest (2 others, same age). The others were fine, active, plastrons were hard and good and no swelling flesh??
This guy has had a tough history though, even had a hole in his leg once for no apparent reason and took ages to heal in isolation/dry tank..
Any possible causes? I feed them all the same diet, all in one tank, no fights, he just doesnt bask, stays in water all the time and i thought that was fine until today.. he cant even swim when i put him back in, he just sinks.. and struggles to rise.
sorry for the essay.. i just dont want him to die.. i lost a baby terrapin once due to faulty filtration which cause an electric current(small) which killed the small one but didnt seem to affect the rest and it was sad..
it's so sad. But i would suggest dry docking him and only letting him in a very shallow bit of water for eating and for only about an hour a day. When I took my RES in to get a check up my herb vet, who also owns quite a few turtles and tortoises, said that turtle can be out of the water indefinately. Some are more prone to stay in the water because they like it more. In your case it sounds like your little guy is just getting too much water and not enough dry time. Have you taken him to a herb vet? If you feel like the vet you go to doesn't give you enough attention or options on how to make him healthier, i would look around for another vet. Call and ask if they specialize in your specific type of turtle. Ask them if they have the equipment to test the turtle there or if they have to send it out. If they are able to test feces, bloodwork, etc. there at a lab, then they are probably more apt. to taking care and knowing what you should do in this case. The biggest key to getting your turtle better is finding the best vet to help you figure out the best solution to make him better. (Once he does get better tho. It might be best to make him come out of the water for a few hours everyday.... just to make sure that this soft shell doesn't happen again.) I'm not sure what the vet will tell you, but i would definately go take him to get checked out by however many vets it takes until you find one that can give you some good advice on how to treat this situation. Sorry if i can't be more help than that. 
I wish you the best of luck. 