Research has been done comparing control groups to those exposed to aquaria.  Benefits have included reports of decreased stress, increase in relaxation,

reduced anxiety, decreased aggressive behaviors and improved eating behaviors and nutrition.  The benefits were seen in the elderly, dental patients and Alzheimer patients, among others.

Aquarium owners have long claimed a feeling of serenity through the watching of various fish.  An aquarium in Connecticut has been noted as having a program to help children with learning and developmental disabilities claiming it helps the kids to develop their social skills through the use of “touch and learn” exhibits in the aquarium.

To learn more, check out this Wikipedia article.