Question

A 250 ml bottle of water treatment liquid will treat up to 625 litres of water. Given that 1 cubic meter=1000 litres, how much treatment liquid is needed to treat a water tank of 2, 240 litres when full many thanks

Answer 1

You will need:
(2240/625)*250=896 ml of treatment liquid.
If each bottle contains 250 ml you'll need 3.6 bottles (3 bottles and a bit more than half a bottle).

I do not think you need the conversion 1 cubic meter=1000 litres unless the volume of the tank is in cubic meters BUT 2240 cubic meters seems too large!