Your problem is simple. You've got too much crap in your tank. I mean literally, your system is filled with fertiliser. It's jacking up your water chemistry. You need to do some heavy duty water changes - large volume, like 50%, or more and clean all the crap out of your substrate and rocks. You've got serious bioload that your system now can't deal with. Tangs are crap factories, and with time will gank an aquarium that can't process their output. Take your rocks out and dunk them in water you siphon out from the system. You might want to do this in a big bucket, or an igloo. Shake/rinse all the crap out. Gravel vacuum the sandbed if you've got the coarser stuff because this is also going to be source. Good luck, you've got work to do.
DJ
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