Possible bad battery?

So going on my third year since I bought crown batteries.
My concern is I have a bad battery
I just pulled gem from winter storage and when I charged it it old read 80% when done charging
So I checked all of battery voltages and they where all within .10 volts of each other except 1 was .5 volt lower.
Assuming that may be the bad battery I plugged my delta Q charger in again and as charging after 2 hours they all ready 15.1 volts. And that one battery is reading 13.1. Not sure why it’s not charging at the same voltage as the others
What’s my next step with that battery.
Should I throw it on a independen 2amp charger for a certain amount of time?
Water levels are all good
There was corrosion on one of the terminals on that battery after I charge s it the first time. I cleaned it before I plugged it back in again

Replace the dud