Now these points are only possibles ..........
Depending on the age of the tank it may have a high Carbonate content in the water due to leeching of the stone in the concrete.
If the first point applies you may need to lower the pH to a reasonable level but you will need a pH meter and phosphoric acid to do it properly.
The third and most obvious point is drink the water and if it tastes nice and fresh and clean you probabluy have good water to brew with.
If the above work out ok -- make a beer and taste that.
Basically suck it and see