The trouble with small tanks with pacu is that it is extremely difficult to maintain good water parameters. It's pretty difficult to change out at least 60 gallons of water (15 gallons at a time) in a 55 gallon tank every day. (We had to do that to keep the nitrates down) I belonged to a pacu forum in 2008/2009 until the moderators' pacus died suddenly at 2 years of age. They had kept their pacus in 75 gallon tanks. High nitrates will lead to stunting and sudden death. Pacu are long lived fish and can live 30+ years. They should not die suddenly at two years.