Regarding nitrate levels and algae - Contrary to popular belief, in a closed system like our tanks, nitrate/phosphate alone is not the main driver of algae growth. Thats why you have tanks that measure 0/0 and have gha explosions and other tanks with 10ppm no3 and pristine tanks. In ocean, nitrates are barely detectable yet algae still grow. Why?
Algae grow when there is "local" excess nutrient and available real estate to grow. Local excess nutrient isn't whats measurable in the water columns, but more like rotting organic matter on surfaces (like dying coral)
In newer tanks, algae tend to grow more with more no3 but thats more bc the rock surfaces are not mature so thats prime real estate for simple life forms like algae and bacteria to take over and colonize first. As a tank matures, coraline algae and other beneficial microfauna take over the surface which prevents algae growth.
If later algae comes back, most of the time its related to some kind of change to the stability that cause those surface biome to start to die, its this dying that gives algae a chance to come back.
As long as the surfaces are healthy, qlgae will have a hard time to grow even with elevated no3.