Ok! First thing you need to do is get your nitrate levels up. The best way to do this is to purchase some KNO3 (potassium nitrate). This will also add some potassium to your water, just in case you're lacking a bit of that too.
You should always ensure your nitrate level doesn't drop below 5 mg/l. If it drops below this you are risking plant deficiencies and algae blooms.
Once you have purchased some KNO3 you can download the Planted Aquarium Calculator from the bottom of this webpage. This will help you to work out how much KNO3 to add to your tank.
You will get algae if you dont fertilise, the algae is probably there because the plants have defficiencys and the algae is winning the battle for the available nutrients, the plants need to start growing again (their growth may now be stunted because of lack of food) in order to outcompete the algae.
