A good way to fill the tank completely with water that has no strange currents is to place a layer of dirt (or something) one meter below where the top layer of water is, then cover the top layer with a smooth sheet of water that has no strange currents. Now remove every single block of dirt under it and the water will fall straight down and fill the tank. The layers below the top aren't source blocks, but this won't make a difference.
If you want to legit-fill the whole tank with source blocks, I suggest filling the bottom layer, then carefully building a dirt floor above it, then remove the dirt blocks one at a time and replace them with water by emptying a bucket against the dirt block next to the block you just removed once you've got the second layer done, repeat at the third layer. This will suck.
Alternately you could try using MCedit, as mentioned in a different solution.
Finally, you could use get a bunch of ice blocks and fill the pool entirely with them (one way to do that would be to use seedscope.net to swap your game into creative mode, fill it, then swap back) then punch the ice until it breaks. Ice blocks will turn into water when broken if they're on top of a solid block or a water block.s