Depends on what else you’re wanting to stock. Bristle nose plecos stay fairly small compared to the standard pleco (4-5” vs 12+”). Could also try Siamese algae eater or if you’re stocking aggressive fish, a Chinese algae eater (although in my experience they tend to start eating food more than algae as they mature). I had a standard pleco in a 20 gallon long and ended up having to put it in my 55 gallon after a year or so due to its size. Good luck!
I would recommend Otocinclus. They're readily available and are great at cleaning up visible algae.
“Even award-winning planted-aquarium experts like Takashi Amano will employ this tireless algae eater. The diminutive Otocinclus Catfish does a great job keeping aquarium glass and plants free of distracting green... the Otocinclus is one of the smallest catfish in the Loricariidae family.”
Source and more on otos:
Very fun little guys and they look great in groups of 5 or more.
(Mods, if you're going to edit posts by adding a source and quotations, better make sure it's accurate. That was no longer a direct, accurate quotation. It is now because I fixed it. Thanks)
Chinese Algae Eater will outgrow your 20L rather quickly.
Plecos grow slower, and lots of people will buy them when they get too big (try relying on a baby Pleco to keep a 110 clean)
Nerite snails are an option, but they're picky. If you just have a small amount of green slime algae they'll keep the glass clean, and they don't reproduce in freshwater. They'll also eat all your dead biofilm.
Siamese Algae Eater does wonders for lots of different algaes, and they're less fragile than Otocinclus, but make sure your LFS doesn't sell you Flying Fox labelled as SAE.
For a great all-round Algae solution without a lot of investment and research, grab a couple Mollies. They eat more species of Algae than snails or catfish, they aren't picky on water parameters (People use them to fast-cycle new tanks), and they aren't anywhere near as messy as a Pleco. If you want the biofilm cleaned off the glass too, pick up a Nerite Snail or two.
Takashi Amano employs and recommends Amano shrimp (really quite obvious) for eating algae, specifically hair algae. I found them to even clean biofilm off walls and do a very good job of it (much like otocinclus), but that was a case where I rarely fed the tank and let the shrimp forage on its own as much as possible.
Otocinclus are another common algae eater.
A single Siamese Algae Eater works well in a 20g long (check out my profile).