units/filters are what produces the RO
Water that you are buying in the store. Depending on the water source your mineral content may vary. If the water is extremely high in minerals when it's filtered you may end up with higher minerals in the water as a result. The membrane typically works in a percentage ratio based on what is already there in the water when it begins filtering. In other words.. if you live in an area with very hard water (what some people affectionately refer to as "Liquid Rock") then when you run it through the RO
you will have a relatively high amount of various metals and minerals. If your water is not very hard and has a low Total Dissolved Solids amount in it then you will as a result have a much lower ratio of these metals, minerals, etc in your produced water.
There are many different sizes of RO
unit measured in Gallons Per Day. Many commercial units are HUGE and sometimes each unit produces thousands of gallons per day but overall they produce pretty much the same thing based on the input water. There are two types of membranes on the market for consumers and they have different productivity and efficiency/effectiveness levels. I do not recall off the top of my head what they are called right now but someone could probably comment on that further.
I'm surprised that water provider only performed RO
process on them since most of the ones I've seen attempt to get ultra pure water that they are not required normally to list the mineral content at all due to it being virtually undetectable by using a De-Ionizing resin. (RO
The De-Ionizing resin strips out the remaining particles that are not neutral in charge since the minerals and metals have a electrical charge on them. You end up stripping most of what is left with the De-Ionizing resin.
As far as using that water for your fish tanks I'd say it's just fine since it's water produced by RO
Process. In the long run it is less expensive over several years to purchase and operate your own water filter in MOST cases unless you have very restrictive water laws in your city, state, province, country, etc It is even more cost effective to filter your own if say you live in an apartment where water is "free" with the rent/lease or have your own water well etc.