Planted tank help!

Question
Hello Krissy.

I am having a problem with my planted discus tank. It houses 5 discus
ranging in size from 2"-4" and a small school of cardinal tetras.

Here is my setup:

It is a 94 gallon tank (L60"xW18"xH20") with an old Fluval 403 (which still
works like a champ..... knock on wood). The intake for the canister contains a
surface skimmer to keep the water surface from developing a film and the
return is below the surface so it doesn't agitate the surface to much. Two
heaters keep the water at a constant 86 degrees F. It is lit by two Coralife
power compact strips each containing two 65 watt 6700k lamps for a total of
260 watts. I also run a pressurized CO2 tank with a Milwaukee regulator
putting out 2 bubbles per second through a ADA Glass Pollen Beetle. The
lights and CO2 are on a timer giving a photo period of about 12 hours per
day. The substrate is Seachem Flourite.

Here is my problem. About 3 weeks ago I did a 25% water change and
syphoned through about 20% of the gravel to remove some sediments. About
2 hours after the water change I heard splashing and checked the tank. All of
the fish were swimming erratically and gasping for air. I took all the fish out
of the tank and put them into a hospital tank where they began breathing
better and swimming okay after about an hour. I tested the water and found
no problems with the ph, ammonia, nitrite phosphate or CO2. I figured there
may have been a problem with the tap water I used to fill the tank. I did have
it in a separate tank heated and dechlorinated before filling the tank with it.
So I did another major water change and put in some small cardinals and they
all died in a 24 hour period.

I then decided to break down he entire tank and start from scratch. Used the
same substrate and filtered water through an Aquarium Pharm Tap Water
Purifier. Ran the filter and heaters for 2 days before starting up things again.
Introduced a few fish after a couple of weeks and again they died in 24 hours!

The pH is 7.0.... Ammonia 0.... Nitrite 0..... Phosphate 0.... Temperature 86F

Sorry for such a lengthly email but I figured the more info I can give you the
better!  I can not figure out what the problem is and I have t say I am very frustrated. Any help you can provide would be greatly appreciated.

Thank you for your time!
Brian

Answer
WOW!!! Honestly, I'm not sure??? Thank you for the detailed information, you're right it is much easier to answer questions with all the facts :)
I don't know off the top of my head how to answer this one. I have some ideas though. I want to be sure I'm on the right track first though, so I'm going to research it this morning and will get back to you with some answers within a couple hours :)

**FOllOW UP**

Stupid question, but where do you keep the food and chemicals? Is there a chance some of the water stabilizers could have gotten too cold or warm and gone bad? Or that something may have gotten in them? It sounds like your tank is very well maintained but have you tested for Ammonia? Do you use town water, well? Sometimes the town will dump a bunch of chemicals in that once built up in the fish can kill it. That doesn't fully explain all of them unless you got them all at the same time which is very doubtful :)Could you check the pH several times a day for a little while to see if it's spiking or dropping? Did you add any new decorations or plants at the time of the original "near fatal" cleaning? Everything I've read leads me to think like you said, there's toxins in there somewhere... Sorry I couldn't be more help.