Thanks mate, that's what I thought about the dead one, but I couldn't find it anywhere. When shall I do W/C? I just want to get cycled and start looking at fish and not water. Lol
I lost a neon that I couldn't find either... I had the entire tank out and couldnt see anything. But my water stayed the same. Just keep an eye on that water and get the levels normalised (and seemingly your tank cycled) before you even think of adding more fish
On not going to add anymore fish unfilled I get the green light from my LFS. the last remaining two neons seem to doing better so I would say I'm on the right path, but we'll see what the tests show.
If you haven't already, do a large W/C right away. When in doubt, always do a water change, because you can never have tank water that's too clean.
Months ago I got a couple of otos for my 55g and one disappeared immediately, the other within a day, and I never found a trace of them. Anywhere. Not even when I completely changed out the tank and switched out the gravel substrate for sand. Sometimes it's just a mystery that's never solved.
I've heard of occasions where fish have jumped out of the tank, sometimes through a gap where an external filter goes in. Even fish you wouldn't expect to jump. Might be worth checking behind the tank, just in case.