Tap water with something in it to stop the algea is fine so long as you don't keep replacing it. Once the tap water had deposited the ions conained within it (which won't be much) you're not adding more so it won't get any worse. Could just buy de-ionised water... some supermarkets have it in 2.5L bottles.

What are you using to cool the water? If you're relying on natural radiation/conduction from the tank 4L is probably not sufficient.