The way you describe it, I would not change anything other than get an ATO to automate your top off and a doser for the No3. If you are diluting the No3 you may be able to even use the ATO as the doser. You would need to measure how much it adds per day to make sure it is adding the right amount but that should work too
Both of those things are on my wish list, but I still haven't figured out how to make room for an ATO. I went as big as possible on the sump and didn't leave room for anything else, and the system is smack in the middle of the living room, so it's gotta be self contained. The doser I'll have to pony up for soon enough.