taken from a site
Stronger: Acrylic aquariums are 6 to 17 times stronger than glass aquariums. This gives acrylic aquariums, especially the larger ones, a wider safety margin when it comes to the possibility of a tank breaking.
Flexible: Acrylic is not rigid...it has "give". If accidentally bumped into, the chance of an acrylic aquarium breaking is much less than glass. If an acrylic aquarium did break, it would crack slightly, not explode.
Clarity: Acrylic allows 92% of light to pass while glass allows 70%. This means you see more of the true colors of the fish.
Insulation: Acrylic is a thermoplastic. It holds heat much better than glass. If you have a power failure, your acrylic aquarium will take much longer to cool down than a glass aquarium.
Machinable: Acrylic can be cut, drilled, routed and machined any way imaginable without loss of strength. Glass loses much strength when drilled.
Seamless: Acrylic can be molecularly bonded...melted together. The aquarium is a one-piece unit. You will not have the problems of leaks or seams splitting, which is an inherent problem with glass tanks.
Cost Effective: Acrylic is less expensive in larger thickness than glass. This makes large aquariums (more than 70 gallons) less expensive. The larger the tank, the better the deal. For example, a 240-300 gallon aquarium can be half the price of its glass counterpart.
Lighter: Acrylic is 48% the weight of glass. Two people can carry an eight foot (300 gallon) aquarium, while it would take four to six people to lift the same aquarium made of glass.