you said that diminished light output = plants not photosynthesising to their potential = algae takes advantage? and my algae situation is odd. only grows on front glass of aquarium as green rather than brown algae. Had some snails before keeping it under wraps too but I have given up stocking them now due to complications if/when I have to use medication.
This is likely because of the present lighting, for two reasons. I don't know your level of experience /knowledge, so I will backup a bit so it is hopefully clearer.
First, algae is natural and normal, and a healthy aquarium will always have some algae. Keeping it under control requires a balance between light (intensity, spectrum and duration each factor in) and available nutrients [I'm assuming plants are present]. Second point is that different types of algae will occur in different aquaria, for reasons not exactly known. I remember the former planted tank columnist in TFH writing more than once how she had different species of algae in different tanks in her fish room, in spite of the fact that lighting and conditions were pretty similar throughout. Be thankful you only have algae on the glass to deal with, as this is easy to remove during the water changes. I always get saddled with brush or beard algae when my lighting/nutrient balance is out.
In your situation, you have a quite strong tube (the Power-Glo), along with a much weaker (the Aqua-Glo is around 1/3 to 1/2 of the intensity of the PG). But both are high in the blue (primarily) and red, with very little green/yellow. So right off, the plants are having a harder time of things. And not surprising, blue light does seem to encourage algae. Tanks that use high blue lighting tend to have more algae issues; I know, I've been there.
Depending upon the plant species you have--some species are faster growing thus requiring more light/nutrients, others are slower growing requiring less--algae may be at an advantage just from the light intensity/spectrum. Aquatic plants will photosynthesize to the max, provided they have sufficient light intensity and nutrients are all available. As soon as any one factor lessens, photosynthesis may slow down. This is referred to as Liebig's law of minimum, a principal of agriculture developed by Carl Sprengel but made popular by Justus von Liebig: growth is not controlled by the total resources available, but rather by the resource that is scarcest, hence the law of minimum.
Assuming you achieve the balance for the specific plants (species and numbers), once the tubes weaken past a certain point the light may become less able to drive photosynthesis, and when plants slow down photosynthesizing they need less nutrients, so these become food for algae. Algae is never as fussy over light or nutrients as higher plants.
Snails help controlling algae, and I consider them useful in every aquarium because they eat all the organics (fish excrement, etc) which breaks it down faster for the bacteria, and thus it gets to the plants. But an imbalance of light/nutrients can easily overwhelm the biological system and only restoring the balance will solve problem algae.
Byron.
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