Here are some of the "funny" shapes I meant. Scoucer doesn't indicate if the aquarium is triangular or a pentagon.
http://www.btinternet.com/~aquariumcity/aquari um.htm
The measurements 30x30x42 would suggest that it's triangular, though. However, I don't understand why you drew another three triangles to make a square, hammerman. If you just draw another one, you make a square 30" x 30". Also 16" = 1.33 feet.
The calculation is now 30" (2.5 ft) x 30" (2.5ft) x 16" (1.33 ft) = 8.3125 cu ft. Divide that by 2 for the original triangle shape and you get 4.15625 cu ft.
Using this site:
http://www.digitaldutch.com/unitconverter/
4.15625 cu ft = 25.888597 UK gallons or 117.69 litres.