Algae will flourish in waters with high concentrations of nitrogen (in this case probably nitrate (over 10pp will promote algae), unless you have live plants where as the plant fertilizer probably has nitrogen in it to help plant growth).
Nitrate levels will always continue to escalate for as long as you have a cycled tank (water changes will only drop the nitrate a little
). If weekly water changes don't help, you may have to change the water anywhere for twice to three times a week if it's that bad.
Worst comes to worst, do a 100% water change to reset it and start over. :S