What do skimmers do?

Question

do i need one for a 20 gallon xpecting 15-20 fish?

Answer 1

It's a bit complicated, but a protein skimmer relies on surface tension between air bubbles and water to attract organic ions dissolved in water. The ions form a film around the air bubbles, which rise into a tube outside of the water chamber and pop, releasing the film in a collection cup at the top.

To reference your specific situation, when you're talking about a 20 gallon tank trying to hold 15-20 fish, no amount of filtration will help. They'll die-off one-by-one until the water can hold enough dissolved oxygen for the survivors.

Answer 2

It pulls organic matter out of the water, it is a nice thing to have if you have a reef tank, but if you have freshwater you don't need one or if its a saltwater fish only tank, it is not a must have.   It is not a good idea to have that many fish in a 20 gallon tank.