Yellow tang losing its mind.....

[MarkoD]

I just did a water change (havent done one in months and months)

i emptied out my sump, cleaned out all the detritus and refilled it with newly mixed water. Left it in the sump for an hour for the temp to rise and then slowly opened the valve to let the water into my tank.

i opened the valve over the course of 10 min. and as soon as it was fully open my yellow tang just went crazy and started attacking my regal tang (they've been together for 8 months with no problems)

it seems like they're all back to normal now.

but why would introduction of new water cause a fish to get aggressive?