Do I Need To Remineralise RO Water If Mixing With Tap Water?

Any time you alter the nature of the water in the tank the fish will have to acclimatise to it. If you start using RO, only use a small amount in the mix the first time, then increase the amount of RO in small stages till you get to the ratio you want. The downside is that from then on you'll have to use RO every time, you won't be able to use pure tapwater in an emergency.

As for remineralisation salts, that will depend on the ratio of RO:tap and the hardness of your tapwater. Using RO will make the water softer, and if it is soft to start with it could end up too soft so you would need to add salts. But if it is hard, so long as you aren't keeping something like Rift Lake cichlids you should be OK without adding salts.

Your nitrate is not a problem for the majority of fish. There are some species that need it lower; researching the fish you have should flag up any problems.

The real downsides to RO are the cost and the necessity of maintaining constant water parameters once you start using it.