Fish Decay?

So I thought to myself, Should I be worried that I havent seen both of my Royal Grammas out at the same time? Well My purple tang came down with ick, and I moved him into my QT tank. Anyways I only have a pair of grammas, pair of clowns, foxface and an angel in my aquarium which have been around since the start and don't mind me moving rocks in the aquarium. Most of them will come up to my hand and play with me when I'm moving the rocks... anyways

Shockingly I found the remains of one of my grammas. (I think... Since I cant find the other one anywhere and inspected every inch of the aquarium) It looked like a decayed fish head and the area by the rock was fuzzy. The last time I had seen both the grammas out was was probably Tuesday... It to me just seems fast for the gramma to decay that fast... My Levels in the tank-

Nitrate - 40ppm (raised 30ppm since tested on the weekend)
Nitrite - 0
Ammonia - 0
PH - 8.0 (dropped .2 since the weekend)

My inverts are all still alive (2 Cleaner Shrimp)and I guess Im lost at this point what to do... I've never had a fish fully decay in the tank. I moved all the remains of what I could find out... and made up water to do a 30% water change tomorrow.

Any suggestions on action? or am I headed in the correct path?

 

I am assuming this is a fairly large tank?
If so, a few grammas are not causing your issue. Small fish like that can be broken down no problem.

*EDIT: this is of course assuming you have adequate amounts of live rock and proper filtration.

 

Its a 120g. with about 140lbs give or take of live rock... I never weighed the 1st two batches of LR that I added but I know I added at least another 100lbs since the 1st two batches because I weighed them. Since the assumed death Ive developed a residue on the surface of the water... Its not oily feeling or anything. But Im assuming thats from the nitrates?