Reasons behind why average fish only live for 3 weeks after sold

Most people just give up after the first fish died. They would conclude "Fish is hard to keep" instead of trying to find out why.

It is not just small children. Most adults don't know anything about fish keep either. They think all they need is a fish tank and a fish. That can't be more wrong. I have to admit until I started to keep fish seriously, I had no knowledge about many basic things such as what equipments I need. I didn't even know what a filter was for. No, a filter is not for filtering debris.