help stingray died, dunno whats going on!

[jason8612]

I posted a bit ago with the fish just dying over the course of 2 year without warning. Today I found my stingray died. I don't know whats going on. I do water changes over the course of the years (every week) and there is nothing wrong with the water. the water is normal temp (84-86F) for the discuses and stingray. Like I said today, he died. theres no reason why he should of died. He ate well with a varied diet, tank water parameters are normal, I did a water change earlier this week, last time I had my hands in the tank was Wednesday, which I washed correctly (first with soap, then cleaned off with water for about 2min) fish do look stressed. I'm going to do a almost full water change, and I'll take a ride up to the fish store tomorrow to see if they can also do a full water test.
there are 2 filters, a sump which is cleaned once a month and a canister filter which is cleaned out weekly from the debris of the live plant, and carbon changed about every 3 weeks. The canister pumps back to a spray bar I made at the bottom of the tank so the stingray always got clean water, plus it moved all the sediments. the fish are fed good with 3 different foods including pellets, flake and frozen. The stingray got live food - worms and ghost shrimp. I also add some chemicals including discus essentials, black water extract, and kent plant chemicals (im 1/2 of their recommendations) plants are doing just fine.
Stingray like I mentioned did not look sick or anything, did not have a curling going on the back part or anything. He did though act a bit frightened this morning and when I came by was swimming to the top a few times then back around the bottom, but I seen him do that alot - swimming to the top of the tank, then just sink back down and continue on.
the other fish are stressed, I can tell, they are really dark (a few of the discus) not sure because of the death or something else. No diseases I can see, all the fish eat.
Only thing I can think of is something, somehow got in the water, and I don't know what.
I'm lost on this one.

[TCTFish]

How many gallons is the tank?
How long has it been up and running with fish in it?
What are the water parameters? Numbers please for ammonia, nitrite, nitrate and pH.
What species of sting ray?

Discus and sting rays have different water requirements from each other. I'd recommend holding off on the total water change until we know more about the water so we can better assist you. Sometimes a full water change like that is not the best approach. Let's first see what's going on. If you already did the change, I'd recommend using a bacterial suppliment like NitroMax or cycle twice a week for the next two weeks. Full on water changes like that can drastically change the water parameters and induce shock. In some cases it's better than any alternatives, but in others may deem more harmful.

How are the other fish? What other fish are in there?