Here is a Wikipedia thing about what is probably the biggest acrylic aquarium ever made;
http://en.wikipedia.org/wiki/Kelly_T...derwater_World
If you have never worked with acrylic you are embarking on a very ambitious first project. My guess is that a 200 litre tank would need to be made of something like 1", maybe a bit thinner. It would be possible to solvent weld it together on all the joints but this is not a trivial task and needs plenty of experience.