My Acei is sick or knocked up; I can't tell.

So my fish have been pretty happy for the past month or so since I got rid of the three male terror fish (a.k.a. male Kenyi). Yesterday morning at breakfast time one of my Acei was just hanging out in the rocks not eating. I thought maybe it was a female and holding (I've had the fish about six/seven months now). Yesterday evening I saw the fish mid tank floating by him/her self. This morning after a good long search I found the fish jammed into a rock crevasse. A few minutes later it swam out and is now kinda sitting on the substrate under a java fern bunch that randomly floats around my tank. The water params are 0,0,40.

Does my fish sound sick or something else? Everyone else in the tank is happy. I can't see anything on the outside that's an issue with the little bugger.

Thanks in advance.

Today is water change day so I'll be doing that soon. I have not sexed the Acei. I tried for 1M/4F. All my Acei have multiple egg spots. I thought that was a m/f rule of thumb. Multiple = male generally. The fish did not eat today either. I have no QT tank to put him/her in. I'll keep watching throughout the day; hoping for the best....

Mystery solved, one dead fish this morning. Changed water yesterday, 25 gallons. I tested my tap water about a month ago and found that Nitrates were 10. I tested the tank water this morning, don't know how long the dead fish was in there, and it's between 10 and 20.