There is no stupid questions, especially if the answer will contribute to a living creatures well being. You may want to contact your local water agency to find out if your water contains chloramine. This is an agent that will cause chlorine to bond and not evaporate. Now, we need to find out what you mean by "immediate". Are you doing a 50% change on a cycled tank, or are you asking if you can fill your new tank with water and chlor-out and then add fish? I've never have problems using it for a tank change. I just add water and the chlor-juice and the fish are fine. ( i do not have chloramines)