The causes of BBA have been debated here and elsewhere quite often. I can only speak to my personal experience, and what I have observed following numerous journals here on the board.
Most cases have much in common. First and foremost is generally dirty conditions. Lack of water changes, lack of filter cleaning, lack of substrate vacuuming, overfeeding fish, overstocking fish, etc. All lead to excess dissolved organics in the system, which BBA loves. IMO, the first step to avoid BBA is to provide uber clean tank conditions. When you see very successful tanks here and elsewhere, if you dig deep enough you will find that tank cleanliness is taken very seriously, and that includes large regular water changes.
Too much light is another common cause. Light needs to be provided in relation your plant types and mass. Combine too much light with dirty tank conditions, and you have a perfect recipe for BBA. Seen it over and over here and elsewhere. The more light you provide the more algae and other problems will expose any weakness and lack of balance. In general, if you are going to blast the light, you better have everything else in good balance.
Plant health is also a contributing factor. Not so much that it causes BBA, but dead and decaying plant matter is a magnet for BBA and other algae. You will notice that folks who demonstrate success pay more attention to every detail. This includes keeping CO2 at optimal levels (and O2 as well), removing any dead or decaying plant matter, pruning old unhealthy growth, providing a good level of nutrients in relation to the plants being kept, etc. No question to me that happy healthy plants are the best defense against all algae.
On a side note, I often see folks reduce or eliminate fert dosing to battle algae. IMO, exactly the wrong thing to do in some cases. Starved weak plants are an easy target for algae. And IME, I have not seen a correlation between rich fert dosing and algae. There are situations where too much is not good, with CSM+B toxicity coming to mind. But that all goes back to general plant health, and providing a good overall balance of nutrients. In general, itís better to be focused on making plants happy and healthy than defeating algae.
Now flow is an interesting factor to me, in that both too little and too much can be a problem. Too little flow and stagnant water seems to be a contributing factor for BBA. Sometimes it is related to too much uncontrolled plant mass, with creates dead spots and a large buildup of detritus. Pull out a large dense grouping of plants, and watch the detritus and organics fly. That dead and dying material needs to be removed, as it leads to more dissolved organics in the system, which you should be trying to avoid.
Now too much flow is another issue. Many times BBA develops right in the areas of the highest flow. Letís say a powerhead is aimed right at a piece of driftwood. Many times, that is exactly the spot where BBA flourishes. I have experienced this and have seen it countless other times in other tanks. Sometimes just reducing flow can beat back BBA quite quickly.
Now these are all just personal observations from my tank and other tanks that I follow. Now as to @Discusluv
, and BBA issues with a an exceptionally clean tank, well I am at a bit of a loss there. My guess would be too much light in relation to plant mass, but really I am a bit stumped. Another possible issue could be adequate bio filtration, as a mature steady bio field quickly converts ammonia. I only mention it as I have seen instances where ammonia spikes triggered an outbreak of BBA, but doubt it is the case there.