Help fish dying--possibly soap poisoning

Need advice on how to save other fish. Husband grabbed a kitchen brush today to clean off some algae (20 gal tank, approx 4 years old). Just noticed three dead fish who were fine last night, so think the brush may have had some dish detergent on it. He had completed a water change this morning and when we noticed the dead fish did another 1/4 tank change. Are we destined to lose the other fish (upside down catfish, striped clown? pleco and gorgeous large spotted raphael)? Should we continue with water changes or is there something we can add to tank to save other fish? They don't seem to be in any distress...............and to think I was just anticipating a trip to the fish shop to buy some new fellas since our Columbian tetras had just passed on over these past few months (old age? They were our original starter fish. Is 4 years tops for the species?)