Yellow blister on ducklings web

[Ducklingmum]

Hi everyone. I am new to posting but have been looking on the forums a while.
I have 7 ducklings about 2 - 3 weeks old and I noticed a day ago that one had a yellowish patch on it's foot. Since then the yellow stuff has spread to the bottom of the foot and a few of the others have it also.
I have put the ok ones into a separate house and disinfected everything. I have some antibiotic spray from the pet shop but I am worried this is spreading so fast!
Does anyone have an idea what it is and hot to stop this in it's tracks?

 

[chickens really]

Are they on a wire bottom cage?

 

[Ducklingmum]

Hi, no they are in a plastic tray that is lined with newspaper. It is changed daily, along with steam cleaning the plastic - obviously not with the ducklings in it!

 

[chickens really]

Possibly the news paper?...use pine shavings....

 

[chickens really]

@Miss Lydia
@casportpony @Amiga

 

[casportpony]

I wonder if it's some sort of fungal infection?