With fish deaths, there normally isn't a "smoking gun." Unless it's an obvious parasite or disease, or an obvious poisoning of the water, folks will never really know what exactly killed their fish. But there are a two things - at least to me - that wave a huge red flag about this tank:
1. You shifted the SG
from 1.027 to 1.021 in one water change.
2. You had 8 fish in a 10g - one of them being a damsel that was harassing the other fish.
It seems like you're still trying to find a reason as to why your fish died... some concrete reason. But to me, the items I listed above seem reason enough. Even though we can't give you an *exact* cause of death, I wouldn't say that no one here doesn't know why the fish died.
Regarding the nitrate test kit, well... someone is wrong. 80 *does* seem pretty high for a tank that has only been set up for a couple weeks, or even a month, but it doesn't seem like it should be zero either with the bioload you had. I'd be more apt to believe you though... since this is the LFS
that screwed you up to start with, right? The water sitting around shouldn't effect the nitrate test. The only thing it *will* effect is pH. You want to do pH tests right after you draw the water.
High phosphates is a little unexpected, but if you've been overfeeding flake food, I could see that you'd have some present. It won't kill fish... at least at levels that we normally see in our tanks. It will slow or stop calcification of coral though... that's why it's a concern in a reef tank. It also fuels algae growth.
For a 10g FOWLR
tank, I'd just let your normal water changes bring down the phosphates. There are sponges and media (GFO
... granulated ferric oxide) and chemicals that you can buy to suck up the phosphates, but if you're not having an algae issue I'd just let the water changes deal with it for now. I'm sure that your LFS
will want to sell you some "phosphate remover" of some sort though.