Do Not get five for a 20 gallon tank. Your newt will not be lonely without others in the tank, it would be happier alone. These are extremely aggressive newts. I had two in a 20 gallon tank, but one was too aggressive. A bite wound on the others tail got infected and killed it. The surviving newt, Tim has been much happier since. For a 20 gallon tank, two is a good number if you are lucky and get docile newts. Watch and make sure that they do not fight. Do not make the mistake I made and let them fight. Separate them immidiately if you see them fighting.
You'll want a screen cover, so they can't escape. All newts can climb, and will escape if able. A screen cover is better than a hood because the temperature will be lower. These newts like cold temperatures from 50-60 degres.
A biowheel filter is great. Paddletails live in streams natrually, so a power filter creates the necessary moving water.
For feeding supply bloodworms, tubifex, or even better blackworms. I keep guppies in the tank too. Do not feed your newt only guppies because vertebrates should not be the only food source, but having fish there is great because your newt will be active hunting.
An island is not necessary but you should have plants that the newt can rest on. Hiding places are required, especially if you will have more than one captive.
Good luck!