Tank Repair

There used to be a product called "Silastic" that was just the thing for that task. It may be the same product as suggested by Sean. If so, it needs to be applied from the inside.

I deal with leaking cement tanks everyday first instant fix, get a few handfuls of sand (this is still OK if it's for drinking, as long as it's clean) and drizzle it into the tank as close to the leak as possible, have a little patience and you will see the leak slow, and perhaps stop altogether, repeat if nescessary, once the sand has "taken up" it should keep the leak at bay for a few days, it will save you an awful lot of water.
Silastic, or a Selleys all weather sealer may help on the outside of the tank if it is a small leak, but unfortunately a good job for a larger crack can only be done by draining the tank. Bladders can be bought, but we generally find the best method to be fibreglass, depending on the leak, the type of tank, and it's age / degradation.

Hope this helps,
Ratbag

Temporary fix would be a product available for leaking concrete swimming pools. Some sort of powder you introduce around the area of leakage.

Not sure about whether water would then be safe to drink tho. Call your local pool guys.

This together with the sand idea would certainly slow or stop it for months.

I have successfully sealed a leaky concrete tank by draining, throughly cleaning and drying it then coating the entire inside surface with a couple of coats of bitumen waterproofer. No taste imparted to the drinking water. However requires draining which is probably not what you need leading into summer.

The repair lasted a couple of years until the incident with the tractor.....