perspex is very expensive, i used some in a joinery job i did & a 2400x1200x9mm was arund $900 per sheet. As for a material for a tank, it is soft & flexible so won't shatter like glass but the softness means it will scratch & become opaque very easily. You can use scrubbers on glass as they are softer than it, my money is on a cleaner scratching it easily. you only need to look at an older style boat with perspex windows so see how bad they can get.