I added too much water conditioner to my aquarium.. is it harmful? Do I need to do a change?
Nah. Too much conditioner won't hurt anything unless you have more conditioner than water.
It depends on how much you added. If you dropped a hole bottle in, I would suggest you take emergency actions, change a lot of the water maybey 75% and add more water, then change 75% of the water again. But most of those products are pretty safe, I beleive that it says on the Seachums Prime bottle that you can add 5X the dose. but i cant help much more as you didnt even say the name of the brand of the product, the size of the tank, the amount you added.
Bigger aquarium and adding plenty of water is ok for more free space but you should mind the pressure in underwater
Don't do that in the future, but it's ok and doesn't require anything now