Will Diatoms (brown algae) just go away once the algae have died or once you've resolved the situation that is encouraging the excess algae? Good Question! I don't think I really know the answer. Each type of algae is of course a complicated living thing and finding details about that living thing can be obscure. I don't know the extent of potential attachement. Certainly given enough time, the remnants of diatoms would break down and disperse but who knows how long that might take.
My own experience is that brown algae comes off quite easily. It easily responds to gentle sponge or hand action on larger leaves, scraper action on inner glass surfaces or removal with brush cleaning on decorations and other removable tank objects. Don't underestimate the power of an old well-cleaned (or new purpose-bought) soft bristled toothbrush. Intricate tank decorations are asking for work, but often regular cleaning will keep them looking good.
Will brown algae just go away as a tank matures? Well, that depends. Since light plus ammonia pretty much defines the conditions for all algae spores to get a start in your tank, it's obvious that cycling conditions (with fishless cycling you have excess ammonia and with fish-in cycling you have short out-of-control ammonia periods) can promote algae. This mechanism is enhanced if the beginner does not understand the control of light amounts or periods yet. Light, of course, is nearly always the factor of primary control available.
Here on TFF, one might make a case that the thing we commonly hear, that brown algae will almost always go away after the tank is not so new, may be exaggerated. My speculation is that there may be a significant tilt toward more mineral content (harder) water on average across the british island that brings us a lot of our members (there are a lot of "ifs" here.) It's just a very rough observation of mine that it is quite rare to see my british friends complaining of super-soft water that doesn't even have a minimum of a few degrees of hardness. Now I'm not saying that hardness is all that significant a controlling factor for brown algae as light but think there are some very soft conditions that can slow growth in vascular plants and perhaps enhance conditions for brown algae.
Brown algae does like to grow in low circulation spots, the exact opposite of black brush algae (for instance.) For example, it's quite common to find brown algae in the corners of tanks just past where the gravel starts or in the crevices of hardscape decorations where water flow doesn't hit as much but where light reaches. By contrast, one might first see black brush algae (little tufts of black bristles) appearing first way out on the tips of pointed plant leaves right in the highest areas of flow in your tank. BBA is stimulated by the ups and downs of CO2 when you do larger water changes of fresh tap water (fresh tap water usually has elevated CO2 for a short period.) Brown algae doesn't seem to have a special relationship to CO2 but like all algae has a tougher time when water changes are frequent because lots and lots of algae spores are lost to a water change.
All this is to just add a bit of detail for beginners so they can see that many of our simplistic thoughts we toss around (and it's important for us to do that for beginners of course as there's only so much one can take in at first) are just the tip of the iceberg as far as real understanding goes.
Anyway, going back to a final, more relevant, comment for marieukxx: physical cleaning and larger water changes are clearly powerful tools. Most important of all though is ensuring that your lighting power is down in that 1 to 1.7 watt/USgallon (ie. below 2w/g usually) (assumes non-high-output fluorescents) sweet range and that your controlled lighting hours are between 4 hours and 10 or so (and assumes sunlight is not adding.) Beginners can usually perform experiments by subtracting or adding 2 hours of lighting at a time and watching the results over two weeks to a month before the next lighting experiment. Once a result is seen, those changes can be more fine-tuned. Others may have even better advice in this area but I hoped this may have helped someone a bit.
~~waterdrop~~
