I came in from work and my tank smelled like sewer water so I immediately looked for dead fish, found one. In changing my charcoal, another dead one surfaced. Two of my five Black Neon Tetras croaked in one afternoon. All of the remaining, 5 Guppies, 3 Black Neon Tetras, 2 Swordtails, all look healthy...but so did these two. They just ate heartily with everyone else last night. No one has looked sick. I tested and got pH 8.2 (typical for my tank), Ammonia 0, Nitrites 0, and Nitrates 0.

I feed very sparingly, did other fish feed on these? They are probably 1.25" at least, including tail, and they have bulk to them. The Guppies are smaller. The Swordtails bigger. What happened?! I'm going out of town in two days... Do I replace the two Tetras to give them peace and protection in their school again? Or do I wait? I'm at a loss. I really don't want to stand by and watch them die one by one.

I change water 25%, twice a week, I feed sparingly, hardly any ever gets to the bottom, and if it does, the Swordtails will get it, usually. My tank is 78-79 degrees. 28 gallons, heavily planted, with each water change I add NovAqua+, AmQuel+, Bacteria Supplement, Freshwater salt, Plant Gro, FloraPride (for one live plant I have).

Thank you all, I don't know of anyone else to ask!
Zambize

How old is your test kit? Just asking because snowboss had an old test kit that was giving false readings.
As for the water changes I'm no expert (which someone like PK will come in and probably correct me lol) but 25% twice a week seems kind of extreme.
I have a 29 gallon and I do about 15 percent once a week and use r/o water. Just seems like a lot of additives going in your tank for 1 plant.

Edit:
Wait is it heavily planted or do you have 1 plant? Confused.

Ah, I didn't realize you were putting so many chemicals in there. First off stop buying the bacteria supplement - that stuff is just garbage. You will grow your own bacteria just fine. Secondly AmQuel has been known to cause pH problems for people so I am wondering if that accompanied with what I suspect is poor bufferin capacity water is what is leading to your pH swings. I would stop using it. Just buy a normal old dechlorinator. There is no reason to be using anything else. That may also be why your readings are so wierd. I seem to remember that AmQuel claims to reduce nitrates in addition to nitrites and ammonia (not sure how effective it is though).

I seem to recall that tetras are pretty sensitive to pH changes. If you are really having swings of .4-.6 in a day, I can't beleive that more haven't died.

Actually, I forgot - you have a problem with high pH, not low pH. The Amquel problems result in a downward spike in pH if you have poor buffering capacity. Sorry for the error. I still am not one who likes to add alot of stuff to the water. It makes everything more complicated when figuring out how the different compounds are interacting. I just add dechlorinator and use petrified coral to raise the pH. Nothing else is done to the water.

Do you have any carbon in the tank. I think I read somewhere that it will remove any excess Amquel and such. Put some new carbon in there if you can. When was the last time you did a water change and what is the pH of your tap water right out of the tap? I wouldn't want you stressing the fish out even more by changing the pH again.

How much were you feeding them? Fish need suprisingly little food. Their stomachs are about the size of their eyes for most community fish so you can imagine how much food they need.