Right I have a 120litre tropical tank.
you can harm your fish with 100% Pure water as they need minerals.
1 week to let the water settle?
that is completely inaccurate, you need to cycle the tank first.
this can take between 4 and 8 weeks to do.
What you are actually doing is getting the right bacteria breeding in the water.
you need ammonia to start the cycle, this can be fish food etc.
once you have ammonia (this is poisonous to fish) you need to get rid of this, This is where the bacteria comes into it, you need Nitrosomonas to turn Ammonia into Nitrites and then a bacteria called Nitrobacter to turn the nitrites into Nitrates.
this is called the Nitrogen Cycle.
plus depending on what fish you want will determnine what PH your tank should be, 100% pure water will have a PH of 5 which is soft and acidic, ideal for oscars etc. now tap water is usually 7-7.5 which is ideal for most tropical fish.
Going back to the cycle you can buy the 2 types of bacteria, I use Tetra safe start and I just throw the whole lot into the tank (100ml Bottle)
Also when you use tap water you will need aqua safe to isaolate the chloromines and chlorines which are harmful to fish
I use 60% pure to 40% tap water this way the fish gets the minerals.
Matt