I think you're using WAY too much dechlorinator. I guess it depends what brand you're using. Did you follow the instruction? I don't know the volume of your aquarium but if it's 2ft long, I'd say it's in the ballpark of 15~20 gal. I'm using Aquaplus by Nutrafin, and it only requires 5ml/gal. I don't know how big the bottle is but half a bottle seems abit too much.

Also, some reading of water condition (ph, ammonia, nitrite) would be alot of help.