Here's a brief primer to help in your research.
The particulate filter takes out (usually) neutrally charged large particles like mud, rust, etc.
The carbon filter(s) will take out some chemicals, but in particular removes Chlorine from water treatment, which is harmful to the RO filter.
The RO filter will take out some percentage of most contaminants in the water, the percentage varying by temperature, pressure, and specific membrane. But for the sake of argument let's say 95% is removed.
Then, if you have one, the DI filter takes out negatively and positively charged ions in the water, hence "De-ionization". Most items going into solution do so by separating out into a negative and positive ion, and this electrical action to remove them takes out a large portion of the rest.
So say you start off with 100 TDS water after it gets through the carbon filters (and let's assume that is all in solution, i.e. "dissolve" solids). The RO filter then outputs water with 5 TDS, and then DI filter takes it to zero (OK, not precisely zero, but most aquarium quality meters will read zero after a DI filter).
If you start with 300 TDS water, RO might be 15, and RODI should still be zero. If it's 600, then RO may be 30, RODI is still zero.
This is why you will see some people use RO water without remineralization, depending on what they start with, and the quality of their filter -- it may actually be "mineralized" enough for them, if they are trying for water that is very, very soft. But it depends on the input water. If you start (as it sounds like you will be) with very low TDS to begin with, a RO filter (without DI) is still going to be darn close to zero.
Water that is too soft, i.e. near zero TDS, is not healthy for fish. It throws off the osmotic balance, in a loose sense sucking healthy stuff out of fish membranes (such flows usually go from high to low concentrations, which is why a RO filter is "reversed"). I have never seen any scientific study to say just how low is safe, but everyone seems to agree that zero is bad. If I were using RO water, with my 250 TDS input, I might not worry. If you are doing so with 60, my GUESS is you would be in the danger range without remineralization.
Now another common way RO or RODI water is used is dilution (someone around here had a signature saying "the solution to pollution is dilution" and that is very true). You can use 50/50 RO water with your tape water, to drop the TDS from 60 to 30 +/-. That works well when you are trying to reduce hardness, but it is marginally effective if your water is high in nitrates. By the way, how high? Is that really the issue?
A natural issue is "why RODI instead of RO" if it takes out 95% of the bad stuff. Well, many of us are a bit compulsive, and once you get all the way to RO you might as well get rid of it all and build your own water just how you want it. But that's not a necessity, but a choice.
Should you decide to do RO or RODI and remineralize, there are other solutions than siphoning. I got a big trashcan (Rubbermaid grey cans is frequently used and accepted as not introducing harmful chemicals). I then bought a pond pump. I toss it into the can to mix (it just swirls the water around). When the can is on a dolly. When I'm ready to put it in the tank, I roll it over, put a hose on the pond pump, and it let pump the water up into the tank. If you do this, you need to get a pond pump that has enough "head pressure" to raise the water to whatever height you need, plus a bit more to have good flow. Many will only go 3-5', some 7-9'. Another alternative if you are handy is a bilge pump - they pump much higher, but cost more, and need 12v power -- I use that as I am moving a lot of water and just plug it into a transformer I got with LED lights. What I did was put a piece of PVC on the end, cap it off, and drill a bunch of holes in it so I get more of a spray when the water comes out than a stream, to keep from disturbing the tank so much.
But... back to the "solution to pollution is dilution"....
If you are over stocked, and building up nitrates, even if your input water has some (let's say 5ppm), the simpler answer is probably to change water more aggressively. If you are changing at 40ppm or even 20ppm, that 5ppm input is not really going to have a huge impact on how much or how frequently you change water.
Think of it this way -- if you change at 20ppm, with 5ppm input water, and did a 50% water change, the new water is 12ppm, then goes up to 20 and change, etc.
If it was zero and you change at 20ppm, your new water after a 50% change is still 10ppm (vs 12ppm above). The input water makes a difference, but unless it is really high, it does not make that much of a difference. Increasing the percentage of change and the frequency of change has a much larger impact.
Now if the input is already 20ppm or something terrible, that's a different story, but that would be rare.
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