I definitely recommend visiting on a sunny day, and combining the aquarium with a picnic in Kasai Rinkai Koen. I wouldn't recommend the aquarium on a really busy day though, as it would be difficult to get a good look into all of the tanks.
Hours of Operation: 9:30-17:00 (last entry at 16:00), closed every Wedneday (changed to Thursday if it is a national holiday or October 1) and from Dec. 29 - Jan. 1.
Admission: Adults:700, Jr. High Students: 250, Seniors 65 and older: 350 yen.
*Disabled, Children under 11, and Jr. High students who reside in/or attend school in Tokyo are free. There are group discounts for groups of over 20.
**Use of strollers may be restricted on peak days.
**Free entry with Grutt Pass 2007 (#47)