Lessions learned on Alage and Phosphates
For plants to work and algae to fail one must be very careful about nutrients in the water - these are needed to feed the plants, but these vital substances must remain in balance and must be removed if levels start to accumulate above desirable values. There are many ways to achieve these objectives but there are far more ways for the balance to fail and then algae to grow.
Unless you have very dense plants (which also means high light/CO2 injection) than you will almost always have high phosphates. This most often has to do with feeding fish in a closed system like a tank and the fact that the tank water does not turn over numerous times per day with fresh water like streams do in nature.
So, first off, I suggest a phosphate test kit (Seachem.) Such a kit gives you a handle on this troublesome nutrient. That, along with a nitrate test kit will help you keep your plants properly fed without incorrect levels of these nutrients occuring, which can often lead to algae growth.
If phosphates get high (anything approaching 1 ppm is getting there in a low density, low light system) than water changes are either not big enough and/or often enough.
If water changes are an issue for any reason (cost/difficulty) and/or large WC's cause nitrates to keep getting too low (bad for plants yet very good for algae growth), consider a small, in tank algae scrubber - mine has dropped my normally 2-3 ppm phosphate level (even with water changes of over 50% twice a week) down to the 0.05 ppm level. I also tried phosphate absorbers but that was $$$ and doing the weekly testing of phosphates levels to determine absorber replacement times was both a bother and ran through test kits rather fast.
Of course, one must feed nitrate if these levels drop below 2-3 ppm in the tank due to a scrubber (not an issue with absorbers) but that is rather easy, and low cost.
Avoid all solid or liquid fertilizers with phosphate since fish will provide more than enough even with a scrubber or absorber (unless you decide not to keep fish.)
This issue has been a battle for me for some time and only recently have I learned these solutions - the hard way.