My 200 gal Tank

[iso]

I have a 200 gal Tank. It has been up for 5 year. All good. Lots of soft and hard corals. Nice selection of fish. The only issue is bird nests always bleach. some right away and some few weeks later. No Idea why. Its very strange. All tests are within normal range and steady. No fluctuation.

Any Ideas?

 

[Rob.D]

What is "normal" range? I've kept BN for about 6 years and I've found it to be the "canary in the coal mine".