Well today I inquired about caretaking for these tanks and was told the one that does is leaving and they were looking for someone to take care of them in the coming future, that includes water changing, cleaning, feeding, and restocking/scaping.
First tank is about a 300-200? gallon long salt...
The cherry shrimp were most likely eaten, the snails should be fine with the goldfish. First of all what size of tank do you have?
The black moor should be fine, but I would moniter for a while after you put him in to make sure he isnt being picked on.
Could be internal parasites or a virus/bacteria. I would treat for both honestly.
Also with what other people said above, a 10 gallon for an oscar is absolutely too small. I would recommend a 75 gallon minimum.