You want a RO/DI unit. It's much easier if you buy the right thing to start with instead of trying to 'piece' together a system. If you buy a decent one it will come with a TDS meter which is the only way to tell you when you need to change the filters in it.
No reason for you not to fill the tank up at this point. You WILL be doing water changes on it and when you add more coral (that will probably come with rock attached to it - you would take some water out at that point.)
As someone above pointed out you need to decide first what kind of corals you want to keep in order to make an informed decsion on what lighting you require. Once you start getting involved in the animals that require higher lighting, you will have to get fans & a chiller (they cost a fair chunk of change..go goggle chiller prices..) and chillers DO produce a large amount of heat when they are working..keep that in mind. Your best bet might be to keep animals suitable for what your system was designed for.
A torch coral..beautiful animals but nope, not good for you tank due to their
feeder tentacles that come out at night. They will sting to death anything they can touch. They also require pristine conditions to thrive.
PH doesn't spike during a cycle.
Even with rock that came from an existing tank, there is usually at least a minimum spike in Ammonia, followed by Nitrite, then by Nitrate..after all that happens & Ammonia & nitrite are totally gone from the tank is your cycle complete.
I'd give it another couple of weeks to be certain especially if you haven't 'fed' the bacteria that should be growing in there. Without something to eat you(good) bacteria will die. Please don't use a fish to cycle! The tiniest (really tiny) pinch of flake food will feed the tank..then test your water a day or 2 later to see if any of your parameters increase.
Oh..and welcome to MR !
