The first question is: Do you need to?
If your tap water is not right for the fish, then you could do a blend with tap and RO, or go with straight RO and add just the minerals your fish need.
Soft water fish (angels, tetras, many others) thrive within a range of parameters:
GH 1-5 is optimum for many, but a lot of them are OK with GH to 9 German degrees of hardness.
KH similar, if the pH is in the right range.
pH 6.0 to 7.5 is a reasonable range for most soft water fish.
Research your fish. If they need something different, then set your parameters to suit the most particular or delicate fish.
Now, test your tap water.
If it falls within the right range for your fish, they why 'fix' it?
If it has toxins or has unreliable parameters then perhaps you do want to use RO.
Run this test:
1) Make a few blends of RO + Tap.
Perhaps 25% RO + 75% tap; 50/50; 75% RO + 25% Tap.
Test GH, KH, TDS, pH. Make the volume of the samples a cup or two.
Which comes closest to being what your fish want, without going over?
If the GH and KH are not at all similar, then make the blend of water too soft, then add back the missing minerals. If the GH is too low, then add Seachem Equilibrium. If the KH is too low then add some baking soda. Just a tiny bit (or else make a bigger sample, like a gallon or two- the minerals need to be added in VERY small amounts)
2) When the GH and KH are similar, and low, and the TDS is low, the pH might end up in the right range. If not you can lower it by running the water through some peat moss. In this test, just add a small handful of peat moss to the sample and stir it around. Give it a few hours then test, and test again the next day.
3) If the tests make the water what your fish want, and they are stable, then you can go into production making special water for the fresh water tank.
Here is how I did it:
Run RO into a 20 gallon garbage can. 10 gallons. Add 5 gallons tap water. Add knee-hi stocking of peat moss. Circulate it overnight. Add 5 gallons hot water (tap) and dechlor. Circulate a few minutes. This made a 50/50 blend at the right temperature for water changes. The peat moss adds the organic acids that these fish like.
Do water changes.
When you are first changing your fish over to this water you want to go slowly. In any one water change you want to drop the TDS (or GH, or KH) by no more than 10% for delicate fish, 15% for hardier fish. It might take a month for them to acclimate to significantly softer water. If your water is only a couple of degrees of GH or KH too high, then one or two water changes may be enough to get close to the goal.
Top off with RO water.