i think you should just put a blue background in, but if you want to make the water blue there is some stuff sold for koi ponds its called pond shade and it just tints the water blue.
lol. this thread.... oh well ill help.
I cant remember what they used, but the EPA did some testing near my parents house 2 years ago. they wanted to see if the stream connected to the bay i think. anyway they dropped in some dye packs(very small) and it made all the water for the entire length down stream green. and by green i dont mean like some peoples tanks. i mean like antifreeze green. pure antifreeze green. it was fish safe(heck they did it just before our local trout hatchery) and eventually disappeared. just sayin. i think it comes in other colors