I have a theory on algae that I was discussing before:
Much like the "cycle" your tank goes through initially, I believe there are many other cycles as your tank matures and eventually becomes a stable enviroment.....due to biodiversity and balances in the food chain.......
I believe that there is an "algae cycle" thats a succession of algae that during time, and as your biodiversity becomes established....feeds and thrives of of the products in your tank.
I personaly believe that every tank will pass though all these "problematic nussiance" algae stages as it matures. However, the difference between the tanks is that as the "algae bar" rises,(My theory is that each succsessive algae is a little more complex and threrfore more difficult to get rid of), the tank must be more "in-tune" by having the correct equipment, flow, correct water, and chemical balances....if one is off and the algae gets what it needs....and thrives....thats why some people have problems with cyano and others don't.....their tank is the perfect enviroment for growing that type of algae.....if you change and correct the enviroment to the "optimal" conditions....by adding the correct clean up crew or not using tap water for instance, then it will go away until you reach the most mature and final type of algae.....bubble(valonia).......In my experience I find bubble usually thriving in older tanks (justin) and stuff like cyano in younger tanks (edwin)........
It is possible for one to not change what is wrong with the enviroment and it to take over the tank in extreme cases.
However, there have been additives like chemiclean found to help people get over that hump and clean up the mess for a time.......but even in tanks that havn't had it in a year......I will bet something has changed....and its the biodiversity level....either something is eating was makes that stuff grow or what ever was making food for that algae is long dead from a sort of "evolution" in the tank. On that note, using chemiclean may get rid of it now...didn't for me.......and your tank may not be maturing at the fastest rate,(who says, tank maturation is a given?), and it could come back and your in a cycle of buying this powder......thats what I would exactly want if I sold that stuff.....a regular customer for it.
I think that all tanks pass through these algael stages.....it all depends on your individual tank and if yours is a breeding ground for it or not if it will become a problem. If it is......fix the problem and it will go away.
The best way to get algae not to grow is get rid of the food or light........its easier to limit the food.......use RO/DI, have enough flow, skimmer, feed every other day(yes every other day.....have been doing it for quite a while and still have fat fish), and do regular water changes.
Yes.....25x turnover is on the lower end, and the difference between 20x and 25x is hard to see a direct corelation of difference by just changing the flow in such a small increment. Upping it from 20x to 40x would be on the order that your looking for, not to mention flow type, thats why people are using Tunzes, Seios or modded MJs........FWIW I am around 67X currently and still have dead spots, some SPS guys have over 100X....in fact thats why I went Barebottom, I got tired of the sand blowing all over the place. (BTW every time I increased the flow, my SPS growth rates increased)