Angelfish, being Pterophylum scalare (or the other two species, P. altum and P. leopoldi) are shoaling fish by nature. They live in groups of several
"Safe" can be open to interpretation. I personally don't favor the use of chemicals to combat green water. Your tank has an imbalance in it,
They spawned naturally. When the fry were large enough to identify and net out, they were moved to the totes.
A rock is a rock. It all depends if the person who threw it had bad intentions or not. A rock is an inanimate object with no emotions so it cannot be
KH, GH, salinity and other compounds combine as TDS, total dissolved solids. These do effect fish.
TDS effects osmoregulation, among other things.