Originally Posted by 919
The rule of thumb is typically one fish per 10 gallons. So you would need approx. 2 fish.
I think you meant 1" of fish per 5g of water. I'm not familiar with the 1 fish per 10 gallons rule.
Have you tested your water parameters? If you have can you post those numbers for us. I understand the want to get some fish in a tank you've had running for a while but you want to make sure that you do it correctly and don't endanger your new fish. Are you wanting to just have fish in the tank or are you planning on doing a mini reef?
A 20g tank can be a challenge so you will need to keep on top of your water changes. I would suggest going to liveaquaria.com and have a look at their "nano" fish section. All the fish they have listed there will do very well in a 20g tank.