blacking out the tank is nonsense. it will not work. it appears as though it's working, but as soon as you turn the lights back on it will reappear. it's just a symptom of a bigger problem-excess nutrients. too many fish, too much food, not enough water changes, tap water...any and all of these things contribute to polluting the tank water. once the nutrient level has built up, things will start to happen, like cyanobacteria gaining a foot hold.
leaving the lights off will not harm the fish. they only need to see to eat.
just use a piece of hose. take the big tube thing off of the end of it.