Something is driving the algae. You may not register any nitrates or phosphates on your tests because the algae is soaking them up before they can get used. Your algae problem is a symptom of a much more serious problem, by using chemicals on the algae you would only be treating that symptom. first of all I would get rid of the HOB
filters and get a good protein skimmer. Don't go cheap either, you will regret it if you do and just end up buying a better one. Simultaniously start an aggressive water change schedual. start changing 15-20% of the water every few days, while you are changing the water suck out as much algae as you can (a toothbrush rubberbanded to your siphon hose is awesome for this). Moniter your nitrates and phosphates. You will notice imense improvement after the first couple water changes. Keep up the same schedual for a few weeks and you will be saying "algae problem, what algae problem?" If your phosphate problem persists then you will have to figure out the source (most likely your water or food).