Well i dont have any personal experience with discus tank, only from what i've read. Due to there slow moving nature discus need a small amount of current but nothing to heavy due to there potential size and shape. Since they kind of just float through the tank they don't need a heavy current.
From your picture it doesn't look as if you have any fish, I would stick with the current from your sump and see how they do, if it moves the water around the tank enough and you are getting some movement from your plants I would say you are fine without it.