Can anyone tell what went wrong? - fish die-off

We have a 10g, non-planted tank, 76 degrees, Whisper filter, 3 julii corys & 3 serpae tetras. Tank finished cycling 2-3 weeks ago, and that’s when the julii corys were added. We use Amquel to pre-treat for water changes (our tap water comes out at about .5 to 1.0 ppm ammonia…)

Went to check on the tank yesterday after work and discovered one dead julii and one very nearly dead serpae. Also had a 2nd serpae that appeared to be injured or sick. The dead julii had no visible marks, wounds, etc. The nearly dead serpae didn’t either. The sickly-looking serpae appeared to have either a wound or disease spot (reddish-looking, protruding spot the size of a bb surrounded by paler skin) about mid-way back on his right side. The other two julii corys and the 3rd serpae all look and act fine. (Prior to yesterday, the entire tank looked fine.)

The only thing new that has happened in the last week or so is that on Monday evening I noticed the water flow coming out of the filter was much less than before. Looking at it, it appeared to be very dirty looking. So, I removed the carbon/floss “cartridge” from its compartment and swished it around in some tank water that I had removed in a pitcher (a new pitcher that has been used for aquarium purposes only). Seems like I had read somewhere here that this was the way to clean one of these cartridges. Got rid of the now very dirty water in the pitcher and added some new water to replace it.

Water tests leading up to this fish die-off and again last night when I discovered it showed 0 ammonia, nitrite and nitrate. (Aquarium Pharmaceuticals test kit w/ test tubes & drops)

Anyone have any thoughts or ideas as to what might have happened here?

AmQuel product masking my test results?...

The tank is non-planted. Water changes have been, on average, every 5 or 6 days since it cycled, and usually at least 50% to 60%.

The serpaes had been in the tank for 6 weeks or so (through the cycle) so any disease problem I guess would have come from the julii's. Unfortunately, we bought them from Petco, so I don't like my chances of getting any helpful feedback from them about that batch of fish. I'll ask them though.

I, too, thought it strange that shortly after cycling, the nitrate readings dropped off to zero. In fact, I'd been considering posting a question about that before this happened.

Do you think it's possible that the AmQuel product is masking/manipulating the test results? I only starting using the AmQuel right at the end of the cycle as I was concerned about getting mini nitrite spikes with the .5 to 1.0ppm of ammonia in our tap water. Prior to that, I know my test kit was working like a champ through the cycle. The ammonia, nitrite and nitrate readings registered, spiked, decreased, etc. just like they were supposed to.

Should I try another type of test kit (or strips) and see what it shows?

Picked up a new test kit at lunch just now and took it home. Bought some of the 5 in 1 test strips. Those results show nitrate of 40 and the lowest detected level (first one above 0) of nitrite. More like what you would expect for a non-planted, newly-cycled tank, right?

Makes me think the AmQuel is ruining the test results from my Aqu.Pharmaceuticals master kit. Does that sound plausible?

If so, what then? I figured I needed the AmQuel because of the .5 to 1.0 ppm of ammonia in my tap water.

Ph is not fluctuating from the tap to the tank - stays right around 7.6 and has since the beginning.

I was in a pretty big hurry at the house over lunch. The nitrate level was easy to see at around 40. The nitrite, though, was a close call between 0 and the next measurement. I should probably do that one again and look at it in some good light before really sounding the nitrite alarm.

The Aqua.Pharm. test kit directions/procedures aren't really that difficult. Shake the bottles, get the right amount of water in the tubes, put in the correct number of drops, cap and then invert the tubes enough to mix it well and then wait. Again, I'd been using this kit routinely from the beginning.

How likely is this scenario: The master kit definitely showed the nitrite level had peaked and was almost down to zero. I think it was around that time that I started using AmQuel on my water changes for fear of the ammonia in my tap causing another mini-cycle. It might be possible that the AmQuel caused the Aqua.Pharm. kit to falsely show 0 nitrite making me think the cycle had ended.