Hiya. Welcome to the forums.
You should be using RO or RODI (reverse osmosis / de-ionized) water. Properly filtered water will be a good first step in many things including algae control (by filtering out stuff algae can thrive on) and livestock health. If you don't have a filter to make the water yourself, there are plenty of options:
1. Get it from your LFS. Just about all LFS' will sell you RO water or pre-mixed saltwater (made with RO). Bring your jugs and fill up! The price varies but at my LFS it's like .40 cents per gal for RO and .70 for salty water.
2. Get distilled water from the grocery store or target. It's the kind used in irons and stuff. It's a little more expensive, like .89 cents a gallon, but this is still cheap for a nano aquarium
3. College student? Many universities have labs with RO water ... they might be willing to share a few gallons.
4. Buy an RO filter. May or may not be practical, but if you decide to, ask around this site, search some posts. Lotta discussion about models and which is best etc.
Good luck!