I was told to only run my light a max of 3 hours per day so that could be your problem right there.
Supra what are you talking about? What are these algae growing goodies that are in the tap water? I've owned a swimming pool service company for 12 years and my company sevices over 200 pools. I think that would make me an expert on algae, and tap water certainly does not help algae growth.
Maybe tap water is different where you live but you can fill an empty pool here with tap water and use no sanitation for 2 weeks and still not have any algae. If you left a pool that had old water in it for 2 weeks with no sanitation it would be a swamp.
When we have pools that have not been winterized and are filled with black water and rotting leaves we do water changes just like with a fish tank...using tap water.
What am I talking about? Are you serious? Tap water is a horrible contributor to algae problems in a tank. If you are into saltwater aquariums and water conditions are way more sensitive, you would find out that using tap water would be your worst enemy. Here are a few examples of this. I believe cleaning pools and home aquariums are 100% different.
Example #1 - Stay Away From Tap Water- Local tap water is often a large source of phosphates and nitrates and other algae causing compounds. Any water you add to your tank or make saltwater with should be purified through reverse osmosis or deionization methods. This will give you 100% pure nutrient free water.
Example #2 - If you want to avoid algae problems, it is a good practice to test your tap water before changes. Likewise, if you already have an algae problem, your first step should be to test your tap water. If the test results show significant levels of phosphates or other algae inducing elements, your battle against algae will be futile until they are reduced.
Most phosphate comes from three sources: tap water, fish food, and carbon. In the case of city water, the municipality may actually be adding it to protect its pipes. In well water, phosphate often come from the leaching of lawn and garden fertilizers.
Example #3 - Green Cloudy Aquarium Water
Usually the result of an algae bloom. The green water will not harm your fish but it is not the most pleasant thing to look at. This happens because of the amount of nutrients and the amount of light entering the aquarium. Your tank water is nutrient rich, which may mean that you're feeding too much, your tank may be overstocked or you're not doing enough water changes or a combination of all the above. Avoid placing your aquarium where it could receive direct sunlight. Direct sunlight will cause not only algae problems but temperature fluctations as well.
To correct this problem, perform 25 percent water changes daily, rinse out or replace the filter media to reduce the amount of nitrates, feed your fish a little less and leave your tank light off for a few days or until the water clears up. If you do practice good aquarium maintenance and still have algae problems, look into testing your tap water. You may be introducing nitrates and possibly phosphates into your tank via the water changes and not even know it. Also look into keeping live aquarium plants which will compete with the algae for available nutrients.
Maybe I'm the only one that believes in using ROI water, but I have never had an algae problem, and I know for a fact I never will using these examples of never using tap water for my source. Its only around $1 a gallon to use ROI water. If you fill the tank with it and do 25% water changes it doesn't cost enough for me to justify using tap water over ROI water.
Not to mention who told you to only leave a light on for 3 hours a day? Thats insane. Does the sun only come out for 3 hours a day and then disappear in the wild where your fish live? I dont think so. I agree with jo3olous on the amount to leave the light on.
Im not saying that the ROI will solve your algae problems like night and day. But if you dont want the algae to come back, ROI is the way to go.
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