Five year old batteries a likely near, or at, their end of life. You should plan to replace them all, soon.
Hydrometer readings of all cells should be at least
1.225, and should show less than 50 points difference between highest and lowest.
An SG difference of less than about 50 points maximum, between highest & lowest reading, may not be critical. In this case a complete re-charge may bring the battery back to useful service
More than 50 points difference, and I would definitely remove the battery from the bank.
You may want to try a cell to cell voltage test, prior to abandoning that battery.
Take a coat hanger, and make two lead extensions about six inches long, and attach them to the voltmeterís test leads.
Touch the positive lead to the positive terminal, and stick the negative lead inside the cell next to it. It should read about 2.1 to 2.3 volts.
Now insert the positive lead in the first cell, and the negative lead in the second cell.
Proceed down the line, until you get to the last cell.
Here you will put the positive lead in the last cell, and the negative lead on the negative terminal.
All the cells should read the same, or within 0.2 volts.
If one reads 3.0V or more, you have a shorted cell and the battery is no good.
If you get a very low reading or a zero reading, the cell is open, and again the battery is no good.
Good luck, and best regards,