Some special aquarium lights can give the water a slight blue tinge but it was probably more likely that the fish were sick and they added a medication comtaining methylene blue. Methylene blue is quite strong and like any medication should only be used if your fish is sick as medication is stressful, therefore obviously you cannot add it just to change the colour of the water. Also never add food dye to a fish tank..... If you really want the blue look check out some aquarium lights but even then certain lights suit certain situations so you would need to do some research before rushing out and getting one. Alternatively you could stick some blue backing to the back of your tank, not quite the same I know but probably the safest and easiest option.