Okay. If you have red algae, you probably have a salt water aquarium, because we don't usually find red algae in a freshwater aquarium. Just in case, your algae looks a little bit reddish, is in a freshwater aquarium, and it's more like a rust color, that's probably just brown diatom algae. That plagues most new aquariums.
If you give the tank a few months, it usually kicks in and you get a lot less brown algae. You'll start getting green instead, which is a sign that the tank has matured. To get rid of brownish, red algae in a freshwater aquarium, you can use algae eaters like otocinclus cats, Chinese algae eaters, and plecos to scavenge the algae. You can also use fresh water snails.
If it's true red algae, like in a saltwater aquarium, it's actually a cyanobacteria. That's telling that you have areas, where you don't have a lot of flow. You have a lot of waste build up, and this bacteria is able to thrive.
The key to getting rid of a lot of these algae's to out compete it for the nutrients that are available. You want beneficial bacteria to grow, instead of the nuisance algae. That's why people use refugiums. Refugium encourages macro algaes and lots of bacteria to grow in an area that's outside the aquarium to out-compete for the nutrients that, otherwise, would be growing inside the aquarium.
So, if you have red algae, and you want to quick fix, you can get some of the chemicals that are commonly available at fish stores like Anti Red or Chemiclean. They're mainly oxidizing agents and antibiotics like erythromycin. The antibiotics will kill a lot of the bacteria, which is promoting this red slime.