Two things you can do: extreme and safe.
Safe: Do a water change , i would do 80% change out.
Extreme: Take all your fish out, put them somewhere else. Take out about 20% of your water and put it somewhere. With water still in your tank, add some bleach. I'd say 2 cups would be good (I don't know how many gallons your tank is, but 2 cups will kill anything). Leave it in for about 2 or 3 hours. Drain your tank completely, and then fill it up with water halfway. As you're doing this, spill the water into the tank by pouring it over each of the walls. Remember, ONLY halfway. Squirt in some declor, a LOT. Leave it for about an hour, repeat this 1 or 2 more times, the last time fill it all the way up and then drain it all. When you're done rinsing it out, add back your original water and then add fresh water, and declor and you should be good.
Bleach is just purified chlorine, so if you just wash it out good with the dechlor it will be fine
