Edit:

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.