Well, here's another puzzler. Nothing jumps out at me.
NovAqua is a good conditioner, I used it for 20 years. AmQuel shouldn't hurt, though once we get past this and the tank is cycled and established I would not use it unless there is ammonia in the source water and you say not, so that's that.
If you have a light over the tank, whatever it is, I would get some cheap stem or floating plants and toss them in. Live plants are amazing at filtering water, and stem and floating plants are fast growers. Even something as simple as duckweed will work here. Some stores have it and it hitches a ride with any fish you buy, so they may give you a spoonful--it will spread like wildfire.
I would also suggest a bacterial supplement. I would always use one if I startd a new tank without live plants. Tetra's SafeStart is one, Seachem's Stability is another. These are both 100% live bacteria, I have used the latter, and they do work. They "seed" the bacteria for cycling, they do not "cycle", they just speed it up initially by introducing live bacteria. Although this sounds like a cycling issue, the tank size still puzzles me with so few fish; but it won't hurt. Don't use other products if you can't get either of these. Just a small bottle, the smallest; only need it for a few days. Do your water change before adding it.
Do not add any more fish until the ones left stay alive beyond a week. If it is the bacteria cycle, it needs time; the supplement will help that.
Kym, I saw you were in this thread, any other suggestions?
Keep us posted, and good luck.