What it looks like
Brown or reddish dusty film on glass, leaves, driftwood, and substrate. Wipes off easily with a finger, leaving a clean streak on the glass. Leaves under the film remain intact — diatoms do not destroy tissue, but they do shade it.
It can be confused with green spot algae, but that is hard and doesn't wipe off, whereas this comes off with a touch.
Why it appears
Almost always — in an aquarium less than two months old. Until the nitrogen cycle is established and plants have started to grow, there is a lot of unused nutrients in the water, and diatoms are the first to occupy the free space. They thrive in low light, so they appear even where there is too little light for higher plants.
In a long-established aquarium, brown film usually means one of two things: the light has significantly decreased (an old lamp, overgrown floating plants) or the plants have stopped growing and are not consuming nutrients.
What to do
In a new aquarium — nothing. After four to eight weeks, when bacteria have colonized and plants have started to grow, diatoms disappear on their own and usually do not return. This is the most common case, and it requires neither remedies nor fish.
During this period:
- Wipe the film off the glass during water changes — it comes off easily with a cloth;
- Do not increase the light "to suppress it": it doesn't bother diatoms, but other algae will follow;
- Do not introduce fish prematurely just for cleaning.
In an old aquarium, check the light and plant growth. If the plants are stagnant, you need to address them, not the film.
Who eats it
Diatoms are one of the few algae that fish genuinely eat willingly. Otocinclus clean leaves without damaging them and handle brown film better than others. Ancistrus remove it from glass and driftwood, but a large Ancistrus can strip soft leaves.
The caveat is the same as everywhere: fish control the film, but they do not negate the time it takes for the aquarium to stabilize on its own.