I only discovered that I'd run out of AquaSafe, the special elixir that removes those nasty chlorines and chloramines from tap water, when I'd already changed the water in my tank. What should I do? Are my fish in danger?
Do you mean you've lowered the water but not added water back? If so just leave it low and get some so you can add water back as soon as possible.
IF you've already added the water back and just discovered you didn't use a dechlorinator I guess you just have to wait and see. but yes definitely endangered them.
You should add the dechlor before putting water in the tank anyway. I always do it before removing water in case I have a problem. Just buy a cheap bucket, fill it with as much water as you need, and check the temp and add dechlor. Then you can pour it in after removing water, and you won't have to worry about any problems.