Unexplaind Fish Deaths

[cbkentucky]

I just woke up this morning and found 8 dead fish in my 29ga tank (3 Bosemani rainbows, 1 Red Irian Rainbow, 2 turquoise Rainbows, 2 Green Tiger Barbs). The only remaining fish are 2 female Red Irians, a baby pleco, and a SAE. The tank is heavily planted, eheim ecco 2330 filtration, Hagan CO2 system, massive light, a FLO rotating deflector on a powerhead and has been running for over 2 years.
I did do a 10ga water change yesterday using Amquel, a little Dr. fish Salt and some Natural Aquarium vital added later.
Tank parameters are Ammonia, Nitrites-0, ph-7.0. Nitrates are deceptive because of the Natural Aquarium vital addition.

Some fish were floating and some not, many with mouths wide open. The still alive fish look fine-no problems.

HELP, I don't know what caused this- All of these fish were show fish- best I've ever seen. THey were raised in my 125ga tank until 2 years ago.

[Jchillin]

Sorry to hear about this unfortunate occurrence

From what you've stated, it would appear that the nitrate was probably the cause as the symptoms point in that direction. Can you determine the exact NO3 level?

This is an aside...do you inject CO2?

[cd5]

This is more of a question than an answer but could one of them died then causing an ammonia spike affecting the others or is the time table too small

[tkos]

I can't see how NO3 would affect the tank in such a short period of time?

But I wonder about the CO2, was there a large output that may have caused the deaths, especially at night when plants are sending out CO2 of their own.

A toxin may have been added somehow. I know that certain ferts say be careful about overdosing, as does Flourish Excell.

During you rwater change you may have exposed a pocket of anaerobic bacteria. It is rare ina freshwater tank but I have heard of its occurance. Did you move the substarte around at all and is it really deep?

[cbkentucky]

I use the Hagan system for CO2- it works for such a small tank and plant growth needs weekly trimming with it. The subtrate is deep- about 3 inches, but I changed the water as usual- nothing new there. With using the natural aquarium vital, it says to multiply nitrate reading by .0009 which would leave me at 0.
I'm still confounded.

[Devilishturtles]

I doubt NO3 would do anything THAT fast. Also, one fish dying wouldn't release enough ammonia to kill all the fish in that fast of time. You said anyways that you tested, and got 0 NH3. I'm guessing as well that there was something wrong with your CO2. I don't know much about it though...