Re: RO water verses tape
OK, here is my story. I used tap for a year. Suffered from hair algae and high nitrates, I also battled with ammonia.
Tap water can be used if your local tap is OK. Generally it is not. Your tap has all kinds of random metals and phosphates in it. It could even have traces of copper in it. As top up water, when the water evaporates from the tank, these metals and phosphates stay in your tank. So as you continue to add tap water, all these bad things start to accumulate and can lead algae and diatom outbreaks.
RO\DI water does not have these metals and typically contain no TDS. So when you mix it with your salt, you know exactly what you are putting in your tank.