Your corys shouldn't have any spots at all. If they have just appeared, it's probably a bacterial infection starting up. It's not at all unusual, especially if the temperature of your tanks is warming up with the season.
I would suggest doing an immediate and large water change and a good bottom cleaning. That and a dose of MelaFix might clear the situation right up if it has been caught early. If not, then you might have to use a stronger medicine. If the MelaFix is going to work, you will see improvement in just a day or two.
Good luck with this. Please let me know if these spots are gone in a couple of days.
water is fine dont have exact results because i get it tested at the LFS
13gallon tank
4 platies and a guppie and the 4 albinos
its not like a spot its just a dark grey circle shadowey area just behind the two front(sorry dont know scientific names!)fins.theyre acting fine no flicking etc and are just playing with the others and eating as usual