Ideally it would be extruded diamond, but that technology will not be available until early in the 26th century. Second would be clear aluminum, but again that will not be available for another 150 years.
The entire problem is that acrylic is not plastic enough, so it fails catastrophically. Well, diamond has all the nice features for structure building, but it's even britter than acrylic. So your sci-fi diamond tank would fail in an even more sensational way.
We have clear aluminium already in the form of sapphire glass (Al₂O₃). Presumably you're referring to a means of manufacturing it at a scale suitable for, say, an aquarium?
