1) The standard advice for low tech tanks seems to be reducing the photoperiod to reduce algae, since algae can be caused by an imbalance of light.
However, I have also read advice that in a heavily planted tank, not enough light can reduce your plant growth so that it cannot out-compete algae. Increased light would then help your plants, and reduce algae growth. I'm sure it is not a simple answer, but what are the factors that would favor reduced light in one scenario and more light in another, and how would you figure out which to do? I've tried a 6 hour photoperiod which has not really helped with algae. Should I try 10 hours, or is that just asking for an algae explosion?
2) Another situation where I have seen conflicting advice. I have high nitrates in my tap water. Thus doing water changes will never bring me below the level of nitrates from the tap. In this situation (again in a heavily planted tank), would it be better to do less water changes to give the plants a chance to eat up the nitrates? Every water change is just refreshing the nitrates.
1) There appears to be conflicting advice in this area because the ultimate goal is not to increase light or decrease light but to seek balance. What is the balance? Well imagine an old fashioned scale with two plates, one on either side. In one plate you we put plant growth. On the other side we put nutrients (carbon, ferts, light). We want both to be in perfect balance. If they are then there is no algae. That does not mean that existing algae disappears, some algaes (Like staghorn) will remain even after it stops growing and must be physically removed or wait for it to decompose (weeks). Others will get eaten by fish/shrimp/snails after its dead if its an appealing type.
In low tech tanks the most common issue is that one nutrient disappears (almost always carbon) while the others remain. Without balance plants either stall or can not grow fast enough to absorb the nutrients (carbon, light, ferts) and this is when algae forms to take advantage of the nutrients. The solution most common for low tech tanks is to reduce the photoperiod. Even better for low tech tanks is to split your photoperiod in 2. This allows the co2, which is likely your limiting factor, to replenish from room air. So if you are doing a 6 hour photoperiod do 3 hours in the morning and 3 hours in the afternoon separated by 4 hours of lights off. Also try to get some surface agitation if you don't have any so co2 has a better chance of entering the water. You don't get a lot doing this, typically around 2-5ppm of co2. But that little bit is all the plants get so keeping it at least that high is important.
I would not increase photoperiod to 10 hours if you are getting algae at 6 hours. Chances are that your plants are growing to their capacity in that 6 hours. I would split your photoperiod and think about surface agitation and try as much manual removal as possible. Beyond that I would need more information before I could further help. A picture would be a big help.
2) It's rare for nitrates to be a big problem in planted tanks. A regular water change helps remove algae because it resets a lot of your tank parameters that might otherwise be getting out of balance. This is especially true in any tank where fertilizer is being added. You may wish to adjust what fertilizers you add to your tank to take advantage of your nitrate level or you may just need to add more plants. But either way it is unlikely to be an issue unless your tap water nitrates are crazy high, like 80ppm high or some such nonsense.