Is it black/brown or green? You need to buy some duckweed and dose (Iron) based on this. If it turns brown add a little more iron during dosages. Anyways you need a Nitrate and Phosphate kit. If you are using CO2 you need to know your kH and pH. Tell me how much light you have, how long it stays on, what you dose your plants with, how often you dose them and how much water you change and when you do it. What type of fish and how many plants you have in your tank and the names if you know them?