What killed my guppies?

[Tiger.]

I had three female guppies; they had been in the tank about a week and a half. I could tell they were stressed because they were clumped in the corner of the tank and their fins were close to their bodies, but I didn't see symptoms of any diseases so I assumed they were just getting used to the new tank. A few days ago, I found two of them dead and the other close to death. I moved the remaining one to quarantine, but she stopped moving unless directly provoked (ignoring food) and died this morning. I knew it couldn't have been ammonia poisoning because I did a water change the day before the two died. The only other animals in there are a dwarf gourami and two zebra nerites, and both seem completely unaffected. The last guppy was heavily pregnant and I thought she was close to giving birth because her belly was squared off, but when she died I examined her and she only had eggs, no fry.

Tank Parameters:
~40 nitrate
0 nitrite
Unknown ammonia, can't test for it.
Temperature at 78°F
Planted (anubias, water wisteria)
HOB filter
Small airstone
Weekly water changes, 30%
Tank has been running for 2 months
10 gallons
Little to no aggression between fish
Water is pretty alkaline, water change water had no chlorine in it

 

[Tiger.]

The last one had a bit of red around her face as well. I don't know about the others

 

[Aiken Drum]

You dont know that it wasnt ammonia, so dont rule that out. Ammonia poisoning takes a few weeks to manifest as ill health and death. So while you may not have ammonia now, it may be ammonia that was present a few weeks ago that kills them.

Also a 30% water change isnt necessarily going to result in safe water. Lets say your ammonia is around 3ppm. Changing 1/3 of the water would only take that down to 2ppm. And if your water is high pH then that would make ammonia more toxic.

Its also very possible they where sick before you got them.