if its just for the overflow, simply remove it and run it under water, squeezing it like a sponge until you stop getting "Stuff" out of it. (watch out for bristle worms etc. a pair of sanitary gloves work well). When your done, squeeze as much water out of it as possible, then shake the living daylights out of it to get the rest of the water out. As long as you get 90% of the tap water out of it you shouldn't have to worry about the tiny amount of chlorine left over. If you're really worried about chlorine (or if this is for a nano) then simply get a bucket of clean RO
water and do the same as above.