First off, what kind of FW tank are you keeping? If it's a fish only system, with no live plants, you really don't even need to be concerned about nitrate, unless the levels get to absurd levele. If your running a FW tank with live plants, then you actually want nitrates, since they are needed for good plant growth. You'll also want to use some additional fertilizers for best plant growth. As a note planted tanks also requite correct light.
Now using RO water is a bit different in FW. The RO process strips all minerals out of it so you need to get them back one way or another. Some people use a mix of tap water and ro water to get the GH and KH they want in their water. Others use all RO water, and use a GH and KH booster to get the water to where they want it. The levels you want for FW tanks depends upon what fish your keeping. However, none of the fish you have are very demanding in this department. For my own planted tank I usually want my water about 4 GH and 4 KH, and the pH slightly acid to slightly alkaline about 6.5 - 7.5.
To boost GH and KH you can use a product like Kent Marine R/O Right but it's a lot less expensive to get bulk products. Avoid the liquid GH / KH builder products because that are especially expensive for what you get. For GH the typical formula for Barrs GH Booster is 3 parts Potassium Sulfate, 3 parts Calcium Sulfate, 1 part Magnesium Sulfate. For boosting KH you can use baking soda.
Here is one site that has a lot of good information and sells bulk products -
https://aquariumfertilizer.com/ I have used them for bulk fertilizers and such.
If your doing a planted tank, the use of fertilizers can be a complex subject, depending upon what your doing and is beyond the scope of this post. I don't usually reference another aquarium BB site here but for planted tanks and everything about the subject take a look at -
https://www.plantedtank.net/
As a note, Reef Sanctuary is mostly a SW reef system site. While a lot of members, like myself, also keep FW, and we'll do our best to help, there may not be the depth of membership your looking for when it comes to FW systems.