Your algae problem is being caused by several thing potentially:
1) tap water, no no no. Not even for top offs, many things in tap can feed algae growth.
2) nitrates and phosphates. Low is not good enough, any amount of phosphates and nitrates period can and will grow algae somewhere in some form. Do more waterchanges.
3) Do not decrease lighting, that doesnt fix anything. You bought the light used so chances are the bulbs are old. After 9-12 months T5 bulbs become more red in spectrum which fuels algae growth. Change your bulbs, its a must.
Fix the problem with water changes and bulb change. Nitrates and phosphates should be undetectable, also be sure you are siphoning out the algae as it contains bound up nitrates and phosphates.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.