Why must you leave your new fish tank empty for two weeks?

It's not completely empty and you don't just leave it running. What you need to do is put it through the nitrogen cycle, which has no set time and can take anything from 2-6 weeks. It's this process that, afterwards, will mean there's lots of good nitrifying bacteria in the tank to keep the water quality healthy. Skip it and add fish straight away and you end up with a tank of dead fish!

There are tons of good websites on how to fishless cycle a tank. It involves a lot more than filling it with water and plugging it in, you need to add an ammonia producing source and test the water every day for ammonia/nitrites and nitrates.

Keeping fish is a bit of a chemistry experiment!