Unfortunately loosing livestock is never easy to digest. Don't give up keep reading and learning. If you YouTube WWC and look at their videos it's clear approach to a healthy tank. water quality is key. I just finished reading your thread, looks like you went full commando with issues you couldn't have seen or known. The mystery fish death in my time was the irreversible effects of chemicals and techniques to catch this vertebrates. when that fish entered a established tank chances of survival in that tank to all animals was reduced to 50%. I would never buy sick fish or anything in dirty tanks. you see contamination is always overlooked in order to save. But your started off GREAT! kinda reminds me in my youth, fresh in this hobby. If you threw a bunch of chemicals into your tank your already going to have that against you. Nothing wrong with starting over. In my time I used the 45 day rule to cycle a tank that I plan on using soon. Be surgical and strict, read up on the fish you're going to get. learn its habits and signs of distress. If you buy from a breeder you will definitely see what I am talking about! FAT Belly's and in some occasions hand fed. it's amazing! Your doing great Marsdahl!