Well you might have too much light with no co2. It all depends on which plants you want to grow. I have 26watts over my mini-m growing HC and its fine. I do have a DIY co2 on there though. You should give DIY co2 a chance. Its really easy to setup up and the only thing you probably need to buy is a $6 nano diffuser off ebay. The rest of the supplies you probably have at home.
Oh, forgot to mention, if its too much light and you are getting algae you can try raising the light up higher from the water surface. I know this might be hard with the light you have though.