Albino Cory Cat Ill ?

[Zojo87_2o03]

hi my albino cory cat has a white growth in the dorsal fin it's literally turned up over night , any ideas ?

My water test results
Nitrite 0
Nitrate 10
Ammonia 0
Ph 7

I do a 20% water change every week and the gravel gets a deep good clean my tank is half gravel half sand, it's 60litres , I feed flakes and algae wafers the fish I have in there are
3 x mountain minnows
3x Amber tetras
1x platy
2x amano shrimp
1x sparkling gourami
1x oto catfish
1x zebra Nerite snail
1x albino cory

 

[fal3]

firstly, how does the growth look? Is it just a lump? Or is it fuzzy? And also, just so you know Cory cats are a schooling species and like to be on larger groups. I understand It might be hard to fit it in a tank that size, because I have a 60l as well.