I am starting a 20L and can't decide how to filter it. I want to stay simple so no sump. I am thinking of only using a protein skimmer. Are there reasons against that? Also what model is popular. Thanks for the help.

i have been running a 20L with just a coralife super skimmer 65 for over 3 years. the skimmer will allow you to do water changes once every 2,3 or 4 weeks instead of every week. note that water changes are also a form of filtration. you should probably look into the tunze 9002 or aqua c remora. they work better and quieter than the coralife super skimmer.

I would use some sort of filtration along with a skimmer. You'll be much happier in the long run.

I also have a 20L with a ac70 and a biocube protein skimmer in the tank. My water is HD.

Only thing is if you decide to go with a bc skimmer plan on cutting 1.5 inches of the tube. Once I did that it started pulling some sick skim.

The more live rock you have, the less you need to rely on mechanical filtration.

As far as a skimmer, some folks SWEAR by them, and others feel like they pull too many nutrients out of the water and have a detrimental impact on corals.

I have a heavy feeding schedule, so I use a skimmer because excess nutrients would be too big a problem if I didn't.

I would suggest, LOTS of live rock and a HOB filter running some floss. A big enough HOB filter will allow you to use it as a mini refugium and grow some cheato which will do wonders for your nitrites and alleviate your need for a sump. There are a bunch of threads about modding an Aquaclear HOB I would suggest reading over a few as I think that would be the perfect route for you.

You won't have much to skim in a "new" tank anyway so you can make that determination as your tank matures.

On my 20L, I had a Nano-Remora Protein Skimmer, a Koralia 1, and a smaller powerhead like an MJ600 blowing across the back bottom of the tank.
It worked well for me.

I think I may go with the tunze 9002 for my skimmer and decide later if I need anything more. This is a new tank, but only as far as the equipment. All of the livestock will be transferred from my 75. I need to downsize in an effort to sell my house. Thanks for the advice guys.

I am not using anything more then a protein skimmer, koralia 2 and LR/LS on my 20g.