Leaking concrete watertank

[Daddy Boots]

My boy has a 6mx2mx2m outdoor concrete water tank that's probably 50 years old and there is slight leakage in a couple of spots on one outside wall. Drained the tank and there's some signs of the concrete deteriorating where the water is leaking. What's the best waterproofing product to use for small areas of treatment preferably a paint/roller on rather than a render as cost of treating the whole wall is too much

 

[HERTS P&D]

What is the tank used for, do you have any pictures?

Andy

 

[Daddy Boots]

Just used for irrigation