got a couple considerations for you;
Make sure you run the ro unit for several hours and let the "good" water run down the drain. You want to purge / rinse the preservatives out of the membrane. You don't want to let this chemical (the preservatives) get into your aquarium.
Next, it's a good idea to test the "good" water coming out of your ro unit with either a conductivity meter or a TDS (total dissolved solids) meter to make sure it's working properly. Of the two, a TDS meter is lower priced. Idealy your ro water should have single digit readings where as yout tap is most likley in the high 300 to low 400's tds. You may want to add a D/I ( de ionization) unit after your ro , it will pick up / remove most, if not all of what your ro let pass thru.
As per water storage, it's good to have a container full of straight ro water (non salted) water for evaporation top off and a seperate container dedicated for mixing and storing salt water. A water jug like what "bottled" water is sold in is a popular container for non salted water storage and use. A larged bucket or small trash can is what most use to store and mix salt water up in. A 32 galon (or smaller) trash can (Clean -new - un used) with a large power head or small water pump (mag drive for example) is a great combination for storing and mixing water. I put a garden hose adapter on the end on my mag drive pump so when I'm done mixing water, I can attach a small section of hose to the pump and pump the new water back into my aquarium.
Hope this helps...Joel