Is dechlorinator harmful to fish?

Generally speaking, no. However, there are some rare, one-off cases where it could be potentially dangerous. The reducing agents in dechlorinator use up oxygen when removing chlorine from the water, and this reaction could be hazardous in poorly oxygenated tanks. For instance, goldfish and discus aquariums can require huge 90% water changes. If you are using water with low oxygen content, adding lots of dechlorinator will further deplete the available oxygen, which can potentially suffocate your fish and beneficial bacteria.

For more information, read the full article on dechlorinators.