How big is your tank?
How often are you doing water changes and how much water at a time? High nutrient levels in a tank, whether from infrequent cleaning or the use of plant fertilizers, will increase algae growth.
Where is the tank situated? A tank placed in any form of direct light (windows, sunlight, an especially bright room, next to a table lamp that is on all the time) will grow algae swift rates.
How long do you have your hood lamp on for? In unplanted fish only tanks, hood lamps should only be on for the duration of a feeding or viewing session, not all the time. Again, increased light will increase algae growth.
Chemicals aren't a good idea, although most people will use them for a quick fix. If you have any inverts, snails or shrimp, chemicals will kill them.
Adding algae eatting fish may also not be a good idea. Again, most people use these creatures as a quick fix without doing the research necessary. Most plecos will grow larger than 12" and require huge tanks, nor do they eat algae as their only food source. The larger a pleco grows, the less aglae it eats, and you are left with the same problems on top of having to find a home for a large pleco. Chinese algae eaters (CAEs) can be purchased very small, but they also grow to 8" and can become agressive. CAEs have been known to parasitize tankmates, sucking on the mucus membranes of the fish's skin as algae becomes less of a food source for them. Siamese Algae Eaters (SAE) are less agressive than CAEs, but can sometimes be difficult to find. Flying Foxes are sometimes mistakened for SAEs, they get to be 8" as well and are good with plants. Otos, aka dwarf suckers, seem to be everybodys favorite quick fix algea eating fish, they stay small and do not damage plants.
The thing to keep in mind with algae eating fish is that once they eat all the algae, their diet will need to be sublimented with fresh veggies (zucchi, cucumber, yam, squash, etc) or formulated algae wafers.
In unplanted fish only tank, phosphate sponges placed in the filter is the best method to help emiminate algae.
No matter what quick fix you use, in the long run, you will still have algae if you do not eliminate the source.
~~Colesea