Platy Is Not Well. Appears Unable To Float..?

[Emily6235]

I have a female platy who is sinking to the bottom of the tank. It appears as though she is unable to float well, and is not deliberately swimming to the bottom. She is still moving her fins, isn't clamped but seems to be "gulping." She is also listless and doesn't react to the other fish. The cories just swim over her like she's a plant. The only difference I've noticed in her before this is that she may be pregnant and had white/clear feces yesterday. All other tank mates happy and well, bright and active.

Only changes introduced were two new platies last night and I also gave the tank a quarter cube of freeze dried tubifex worms.

Tank:
20 gallons
10 neon tetras
3 false julli cories
3 (now) platies

ammonia 0 ppm
nitrates 5 ppm
nitrites 0 ppm
ph 7.5
temperature 76 F

heavily planted

 

[#1 Fish Fan]

Pregnancy doesn't usally cause fish not being able to float. It is not likely it has to do with pregnancy. Have you noticed anything different about the way she looks/behaves other than not floating?

 

[Emily6235]

She is still hanging in there but seems to be listless overall. She no longer appears to be sinking, but does still spend her time hiding on the gravel or in a corner at the surface. No appetite either. Feces are back to normal though. All other fish in the tank still seem healthy.