On Friday at around 10:30AM my wife called me and told me the tank was very cloudy. A couple of fish were lying on the sand bed........NOT GOOD. She sent me a video....I walked her through a few things and couldn't figure it out. I instructed her on how to do a 30 gallon water change....(just opening and closing some valves). I ended up leaving work and going home by around 1PM. On the ride home I thought about my doser and if that had overdosed anything. I use a Pacific Sun Kore 5th Doser and dose Potasium, Vinegar, E.S.V. trace Elements and Magnesium. Well as soon as I got in my fish room I noticed ALL of the trace Elements had dosed into the tank. I only dose 25ml per WEEK but the container was full (60 Fluid Ounces) so it dumped around 1775 ml into the tank in about 3 hours. I called and emailed Bob at E.S.V. and he was very good in getting back to me. That overdose sucked all the oxygen out of my tank and damaged the gills on specific fish. I ran extra oxygen into the tank with those portable aerators and tried to do massive water changes while also running carbon. I lost ALL of my Tangs.....2 yellow tangs, 1 purple tang and my 11 year old sailfin tang. All the other fish and all my coral are fine thankfully. This was probably my fault.....about 2 weeks ago I noticed that particular one doser was not dosing. When I finally got around to testing it I found the problem was a clogged check valve. I cleaned it and set it back into the tank. I suppose I should have re-calibrated that channel but I didn't and my poor fish paid the price. It would be beneficial if the doser had a fail safe built in....such as either a timing mechanism or some sort of measurement that would shut that doser off in the event it runs too long or doses too much. I'm never going to leave more than 2 weeks worth of fluid in any of my dosing containers anymore. I'm not happy in loosing my tangs but I am grateful that I didn't loose ALL of my fish and ANY of my coral. SUCKS !!!!!!!