State of Water in Water Butts

I have a very large water tank which is, despite the drought, completely full and, now there is to be a hosepipe ban, I will need to use this water.  The water has been collecting since before Christmas.  Can anybody tell me if this water is likely to be okay as it is for use on vegetables and fruit trees?  Is it absolutely necessary that I apply treatments such as Biotal, which I would rather not do?  Or is there a free to prepare and natural solution?

I would use it, but I don't grow Veg or Fruit. Can't see that it would cause any problems

Yes it will be alright to use on vegetables and fruit trees as long the water isnt slimy or green , I am using  water out of my water butt, I am also using wate from the washing up and from the bathroom for bushes and flowers. 

Thanks for the responses, v. useful.  I have got a little green growth/slime on the surface of the water.  Is the use of Biotal or similar the only way to get rid of it?

When we had the drought in the '70's, we channeled the water from out washing machine and bath into a but and used it for watering the lawn, I suppose it was the soda in the washing powder, the lawn turned a beautiful rich, very deep green and the flowers were fantastic that year.

I have used water from butts for years although I do tend to empty the butts each winter and get the leaves etc out of the bottom but i've never had a problem when I haven't done it either.