Cool, so you have a 20g tank with 36 watts of CF lighting and a 12 hour photoperiod...now we are getting somewhere
For starters, I think you are leaving your lights on too long. Most plants do fine on 8-10 hours of light per day. Also, in my opinion, you are probably on the verge of needing CO2 or an alternative carbon source (Excel or Glutraldehyde). I'm not very familiar with your light setup but 36w of CF lighting can get a good amount of light into your tank, depending on the quality of the reflectors.
Do you have a lot of fish in the tank or do you overfeed? Since you are not adding any fertilizers, fish waste and food are the only source of fertilizer for your plants. If the plants don't get enough "food" they stop growing. When the plants stop growing, algae starts.
Also, fertilizer will not reduce the algae once it is present in the tank. It will do a lot to prevent it from forming but won't eliminate it. Algae use the fertilizers just like plants do. You will need to get in the tank and manually remove all of the algae you can. For filamentous algae (Hair, Thread, etc) a toothbrush works great. Once you have as much algae removed as you can, you can overdose with Excel (See the thread in the sticky section at the top of the Algae Forum) to help eliminate the rest of the algae. Make sure to read that thread as there is a lot of info on plants, fish, and shrimp that can be sensitive to Excel overdosage.