dead of unknown causes

[guppygirly]

I went to feed my fish this evening and found a dead cory. I immediately checked the water parameters and everything is normal. The only thing above 0 is the nitrate which was at ~20...not too high according to the test kit. There were no symptoms...Any ideas? My tank is a 29 gallon US) with the following fish:
3 rams (very small)
2 pearl gouramis
2 dwarf honey gouramis
3 otos
3 white skirt tetras
6 white clouds
5..now 4 albino cories

I realize that I am fully stocked, but I didn't think I was over stocked. I have plenty of live plants, a grapevine, and some dark slate for hiding in. I have a regent 30-60 aqua-tech filter and I change about 20% of the water weekly. The tank has been set up for about a month and a half. It is fully cycled, I started with an already colonized filter and large rocks from an existing tank. I feed a variety of foods in small amounts once a day (alternating days between flake and granules, hikari frozen brine shrimp once a week, sinking algae waffers every other day). The last fish I added was a week ago (the otos), which came from my healthy 10 gallon.

Just trying to give thourough information. I'm at a loss as to what has caused this death.

 

[noop]

sorry to hear about your cory. i also lost one, last week. came home to find him lying upside down on the bottom... have no idea what caused it, the other guys were fine

 

[Cheese Specialist]

That is a shame gg.

Just thought I would bump you up and maybe you'll get some help.

 

[crimsontsavo]

Is it possible he died of old age?

 

[guppygirly]

Could be old age, he was bigger than my others. Who really knows how long some fish sit in the store before they are bought... One other question. The cory I've had for a while looks more cream colored and has an irridescent sheen, the newer ones look very white and almost powdery. I always assumed I "burnt" the former by using salt on him (I didn't know about that when I first bought him... )