I had a very simular experience mate.
I bought a pair of clowns and one did not look right after a couple of weeks. A kind of shimmering but not enough for me to say it was brook. I gave it meth blue dips and it did shift the shimmer but it came back.
Eventually i found it dead. I think it was weak and the B@***** of a hitchiker crab dragged it in to its layer.
The other one was fine throughout and has now paired with a very small male i bought for the nano.
Sometimes i think things just arnt right.
Top tip - keep checking your refractometer is calibrated. Being a noob i didnt think they needed it - over a peroid of 6 months i dropped to 1.015. I had a very very near miss as i didnt know what was wrong. I lost a cleaner and some of my Zoas are looking very bad.My hammer is starting to recover and the leather is nearly back to normal.
Best of lusk with everything.