if it is bacteria bloom the best thing is to just wait it out, carbon won't help that. It is usually just a week, but I have seen it take a couple before.
The usual instructions on dechlorinator is to add enough for the volume of the tank to the water, then put in the tap water. That is the instructions from Prime, which is what I use. If you do it backwards with a large change it can cause problems. With a healthy filter and smaller change the chlorine is to small a concentration to matter, but I'd still do it the proper way.
46 gallon bowfront, Dalmatian Mollies, Swordtails, Otos
75 gallon Lake Malawi African Cichlids
3 gallon nano desktop with Betta at the office