worried about disease

[bunnycutie1220]

ok..i have a 10 gal with 2 platy, 2 guppy and a feeder goldfish.(ok i know im not supposed to keep goldfishes with them my mistake when i started so dont criticize) for the past week or so Goldy hasnt be eating well. she just stays in a space like floating doing nuthing, and today i found her dead . no symtoms, other fishes fine(eating like pigs), temp 23-26.what i did notice (after she was dead) was white scales on the bottom of the tank, could this be dropsy(i dont think so)? once again...other fishes perfectly fine. or is it just because they are feeders and they dont have a long lifespan?(ive had her for 4 mths)
dont ask for ammonia/nitrite i dont have test kits but im sure they are 0.

 

[WILDER]

Could of been a bacteria infection, but white scales can be columnaris, and if they fall off she could of scraped them off on the gravel, sorry for the loss, R.I.P. little guy.