My understanding is that it is always best to adjust the acid before you ferment it. It is also my understanding that it's difficult to check during the fermentation. I believe you can make adjustments afterwards, but it may effect the flavor.
You may also get a malolactic fermentation...
I made Rhubarb a couple years ago and it had an extraordinary amount of acid in it. As I recall, I added Calcium Carbonate to bring the acid level down. I would think that would account for the tartness. Especially if you added Citric acid on top of that as the recipe called for. Did you...
I made Elderberry wine last year and had the same thing. as I remember...I think the only thing that will cut it up is vegetable oil. If not Jack Keller has the remedy on his blog