What do I do about algae in my 10 gal. aquarium?

I've had it for about 4 yrs. and only cleaned it about once a year. About 5 mo. ago I started getting an almost black algae. The last time I cleaned it, I replaced almost everything in it, filled it up and really doused it with algae killer and let it sit for 3 days. Then I emptied the water added fresh with the recommended amount of algae killer and the other water conditioners. Within 3 weeks the algae was back, so I called the fish store and they recommended snails. At $3.00 a snail I bought 3. One I never saw again. One died and the other is all over the place, but can't seem to catch up. I only keep the light on a couple hours a day after leaving it off for 3 days and covering it with a towel to keep any light out. I was out of town for a month and had a neighbor feeding them when I came home to an almost black aquarium. What happened and how can I stop it?