Wicked deal, well done. I started my first ever reef tank 3 months ago now. I asked quite a few questions here and they were answered very well by some people here. Some of the reefers on here are very helpful, and realy know their stuff. One thing i wouldnt reccomend doing, is topping up with tap water. Get new filter stuf for the RODI unit that comes with it, and you will be glad you did. My tank has had the most chronic brown algae (covers EVERYTHING) since when i started using tap water, but i am buying a DI unit now so hopefully things will get better!!
I use water from the sea, only because i dont have lots of RODI water to use for ASW, and i dont wana buy salt mix :lol: NSW (for a small tank) is very easy to get. For my tank, i only need 30 odd Litres, so i just go fill up two 20L water containers, chuck them back in the boot, then take them home and do my water change.
You will get hooked i warn you now!!
Good luck!