Zebra danios dying

[Yellowfire]

I have a 40g breeder tank that I got about mid october. I fishless cycled the tank so that I had (at least what I thought) was 0 ammonia, 0 nitrites, and about 10-20 nitrates. Did a 24 hour test and saw the conversion happen from the pure ammonia to nitrites, to nitrates.

I then did a large water change and at first I added 10 zebra danios. Everything was fine for a week, so I then added 6 black skirt tetras. Everything at this point was still great. Water looked good, wasn't having any problems.

Just about a week later (so almost 2 weeks since I got the fish) I woke up and was sad to see a half-eaten danio corpse in the water. I was slightly confused, but I just took it out, tested the water to make sure everything looked okay (it did) and chalked it up to just a poor quality of fish.

It's been about a week from that day now and I've had two more zebra danios die. I currently have 12 black skirt tetras in the tank and they all seem to be doing really well (they all look really healthy and active and go crazy when I feed them). The danios typically look active as well. They are constantly chasing each other it seems like, which at first worried me (and still may be a problem?) but I have read that this is simply what these fish do...

I have a few theories about why they died, although this is my first tank so I don't have much experience in terms of successfully keeping fish.

1. It appears that I have ammonia in my tap water. I have the master API liquid test kit and it is not as green as .25, but I don't really think I can say that it's the bright yellow it should be for 0 either.

2. I am using water out of my tap which comes from a water softener. I have read conflicting opinions about whether or not this is harmful to fish.

3. All the danios that died were rather small. The store I was buying them from had a 5 for 5 deal so they sold through all their larger stock quickly. Perhaps they were being harrassed too much and just died from being tired? Can zebra danios actually do that?

4. Do fish just sometimes die no matter what you try? Could it just be that it was a weak batch?

Thanks for your help. Each time I wake up in the morning I now count the danios thinking that I will probably have lost another one...

 

[Yellowfire]

I forgot to mention it, but one of the zebra danios started looking a lot darker yesterday too... not sure if this is normal sometimes or if it is a disease or a sign of bad things to come....

 

[Yellowfire]

Another idea: I read a thread saying that a person's white fin tetras were bothering their corycats enough to the point that they were unwell and actually dying. They would only do this when the lights were off and thought nobody was watching, though. This lead me to think that perhaps my black skirt tetras were harassing my zebra danios. I am at school right now (yay finals...) so I can't test this theory immediately, but I will see what happens when I get home.