Any healthy tank will have algae in it. Some algae is actually a good sign when it shows up because it means you have good clean water to support plant life. However, it is not always cosmetically the greatest and yes, it does mean you have slightly higher nitrites but again, any healthy tank will have some amount of nitrites. It would actually be strange and not a great thing to not have any.
Some plecos can make good algae eaters but depending on which species you're getting into they can get rather large and eventually need a lot more than algae to sustain them. It's still a good idea to have a good bottom feeder though, to help get rid of the bits of food, that may accumulate on the bottom, and to clean up that unsightly algae. Even a plant using up the nitrites would not be a good means to remove the nitrites. You would still need to keep up a good maintenance routine, and the plant really only becomes a good nitrite harvester when it's got sufficient amounts of light, co2, etc. Not really worth the while in teh long run, when a weekly water change can eliminate the problem at the source.
Ancistrus species of pleco make good cleaners and a good choice for a 30g tank because of their small maximum size. Other good choices might be either mystery snails or chinese algae eaters, but that second one must be treated wiht caution. They are incredibly industrious cleaners in thier early life, and grow to be on average around 9 inches wiht more aggressive adn carnivorous tendencies.
HTH
