I'm still sort of hung up on the filtration, or lack there of. I saw that there are 4 2" (maybe? hard to guess) holes in one side of the tank, but found no pictures that showed what was on the other side.
There were 2 pics that showed anglefish eggs and a fluval 305 in the background, but thats pretty clearly a different tank.
Looking at the timing of posts there is no way the concrete soaked long enough to not spike PH to untold acidity. Its likely the fish are ceviche..