colony acting funny

[Jritter02]

What would cause a colony to suck itself in? I checked my water today and all paramaters are good except nitrates. They were at 40ppm so i did a 50% water,change. I have a 24g nano. Just a clown,fish and tang (no need to throw in comments about tje tang needing a bigger tank, i already know this and thats not the reason im here). I haven't changed lighting our anything like that. This colony used to be full and bright up to about a week ago. I have 3 other small colonies that still look awesome. Up until a week ago though this was the best looking colony i had.

 

[650-IS350]

I know your saying Param's are good but what is your
Alk, pH, Salinity at? When was the last time you changed your bulbs? or age of the bulbs as spectrum does shift if I remember as bulbs age.

Have you closely watched your tang? They are KNOWN nippers of Z's and P's.