I've had fish in my aquarium for 3-4 weeks now. Yesterday morning I did a water change which turned out to a much bigger change than I expected - it was around 80% since the Python sucks up so much water while you're trying to vacuum the sand. I moved decorations to clean under and around, which scared some of the fish so they were all in a corner. Got it filled back up and de-chlorinated, but I noticed a few of my bloodfin tetra were just laying perfectly still as if in shock from the changes. But eventually everybody seemed back to normal again. Everything was fine again during morning feeding, but when I checked about 3 hours later I found one of the bloodfin tetra dead against the filter intake. I removed him and tested all my levels and everything seemed normal. I did detect a hint of ammonia so I put some Tetra SafeStart in. I don't know if I did something wrong with the water change yesterday or if these are circumstances out of my control, but I still feel a little guilty.