Improper tank size:
Please give your fish tanks of the proper tank size, tanks should be wide enough for fish to easily turn around without touching anything, and at least 4 times as long as the fish.
Don't keep your pacu in 325 gallon, they need at least 1000 gal, same goes for redtail catfish.
Oscars shouldn't be kept in a 55 or 75 gal, instead a single oscar needs at least 125 gallons.
In to small of tanks Waste builds up quickly , harming your fish.
Spinal problems occur from not enough space, and abrasions occur more frequently , fish can also show maladapted behavior such as jumping out, hyper aggression , lethargy etc