There are several reason RTN occurs especially if its a new piece placed into your tank. Meaning if you already have SPS in your tank that's thriving and no sign of RTN i wouldn't say its your tank but a combo of things such as:
Being it was Brown already then coming from a totally different tank parameters then to yours the stress of your parameters including lighting (which are fine if you have existing SPS) could lead to RTN. RTN is a bacteria but your corals can also eject ALL its Zoo from its tissue if when introduced to a NEW environment which can also seem like RTN.
Corals that are going through RTN or STN have me dipped in Coral Rx, Lugos, or a Iodine Dip to slow down or stop the spreading then removing the bleached area (prior to dipping) is also needed.
I've saved overall more than i allowed to just die using this process, when its bacteria the success rates for been high but when its lighting its been low survival. That's my 2 cents i'm sure there are others with more knowledge and experience than me. hope it works out.
