Most shelving materials as well as paper itself floats, so bookshelves can't be anywhere near as heavy as an aquarium even if you have them packed impossibly dense.
Some one deleted their reply to you listing densities of paper and water. Here are some figures from Wikipedia
- The density of paper ranges from 250 kg/m3 (16 lb/cu ft) for tissue paper to 1500 kg/m3 (94 lb/cu ft) for some specialty paper. Printing paper is about 800 kg/m3 (50 lb/cu ft).
- Water: 1000 kg/m3
So they are not that far apart when you consider that the average bookshelf is often bigger than the average aquarium.