An RODI unit is one of the most important things you can get for your tank -- using tap water can lead to all sorts of problems.
You can find a decent RODI unit online for $150 to $200. Most of them get plumbed into the cold water line underneath your kitchen sink, but if you don't want to do that, you can buy a faucet adapter for most units.
A good RODI unit will have at least 4 stages, 5 or 6 stages being preferable. The stages indicate how many steps of filtration the water goes through. RO stands for reverse osmosis, a method of filtration. DI stands for de-ionization, another method of filtration. Once tap water goes through the RODI filters, it should be 99.9% pure.
If you buy an RODI unit, you will also need to buy a TDS meter, which measures the total dissolved solids in the water. The readings from the TDS meter will tell you if your RODI unit is working, and if your filter cartridges need changing (usually once a year). TDS meters cost $15 to $20 online.