How many gallons or litres is the tank.
-20 gallons
How many fish and which type.
13 fish, 3 mollies, (one effected with this symptom) 5 white clouds, 2 loaches, 2 swordtails, 1 tetra (blackskirt)
Water stats in ammonia, nitrite, nitrate, and ph.
-0 ppm ammonia
-0 ppm nitrite
-20 ppm nitrate
7.6 ph I think
Is the fish bloated or skinny.
-skinny, (normal size)
What does it look like when the fish goes to the toilet.
-n/a
What do you feed the fish.
-Crushed up flake food with freeze dried blood worms
Any fish been picking on the fish.
-Not that I can tell
Can the fish maintain it balance in the water. Does the tail look like it weighing the fish down in the water.
-No he cannot maintain his balance in the water, it does looks like his tail is weighing him down, and he's swimming almost sideways like.
EDIT:
Sorry I was out the whole day,
I just looked in the tank, he's now at the bottom of the tank, sitting sideways, (like when their dead) but he's still trying to swim)

What could have caused this? I don't want this to happen to any of my other fish,..