RO, Tap water or rain water, whats best to use.

Before we look at these, a fact many hobbyists do not know or don't think is that large of an issue is that our aquarium pets drink the water they live in!!
Anything in that water gets into their system and quite often their blood supply.
Saltwater fish have to drink the water that surrounds them in abundance to be used to off set osmotic absorption of the salt in that water, the same as us, though we take a bottle of water with us that hopefully has no pollutants in it, but if stranded floating in the ocean, just like fish, our body works hard at repelling the salt trying to enter our bodies through our pores by using what ever fresh water there is with in us and that, if left long enough, kills us via dehydration.
So you can imagine the main killer of our marine aquarium pets is what ever is not the mix of sodium chloride, hydrogen and oxygen and a few needed trace elements that make up of pure saltwater enters our fish via constant drinking and in two days or after years always kills them!
Fish have ion pumps in conjunction with their gills and a little is done by their kidneys of removing that salt to make freshwater, but those do not remove any other impurities from the water!
 
To answer anything like this you need to know what each is comprised of.
Tap water-
To make this water we drink sort of safe for human consumption there are water soluble metals and chemicals added to keep bacteria from developing, to stop pipes from rusting and keep seals lasting longer, all are extremely toxic over time to us and marine life.
If you add water ager or neutraliser to the tap water it can be used soon after that, though there are still less harmful substances in it that will harm all marine life, not quickly these kill over months to years.
With a reef tank, if you have a clean bio sink area externally and a sizable algae importing area with racemosa and chaetomorpha algae's in use, then neutralised tap water is fine to use for top ups but not for mixing your own saltwater.
The bio sink will neutralize all water soluble metals and the algae if done right which near on no one does, will take out all that is left to give you great saltwater.
Its not worth it, do not use tap water for saltwater tanks.

Rainwater
Rain water is high in nitrate created by lightning and initially picks up pollution in our air.
Rainwater can be used quite well for evaporation top ups or the basis to make your own saltwater but for freshwater life using a lot of it, it is completely free of any valuable trace elements.

It is usually PH neutral as in roughly 7.0, it should not be caught as the rain starts, there are toxins in our air it is capturing as it heads to the ground, you should begin catching it a few minutes after it starts raining not as it starts.
If from a water tank, if it is made of concrete it will have slightly elevated PH due to the lime in the concrete, if it is a plastic tank there are UV protection chemicals leaching into the tanks water constantly and if your home has metal roofing, there may be anodised products coming off it always with rains.
Tank water is quite good but not perfect!!!!

RO or with DI resin
DI is not essential at all!!!
RO meaning reverse osmosis, if all three cartridges are kept changed when need be, this filter system will provide you with the safest evaporation top up or for making your own mix of saltwater.
The cartridges comprise of -sediment, then a carbon cartridge to get out some chemicals like chlorine or chloramines then the RO bladder cartridge.
The RO cartridge, being the last if no DI resin one is in play, is called a bladder, because the lining of this one is like a plastic bag.
If sediment is left too long the flow will slow, if the carbon one is left longer then 4 to 6 months the chems that get past will eat away the more expensive RO bladder material and now that filtered water is not so good anymore. If the carbon is changed at proper intervals the RO will last 3 to 5 yeas with out replacing
For the speed and volume at which the water is filtered, check with the supplier of it all as to how to set the flow.