Brown Algae in a New Tank: Diatoms and How To Beat Them

If your tank is a few weeks old and a brown, dusty film is creeping over the glass, the sand and the leaves, take a breath. That is almost certainly a diatom bloom, and it is one of the most normal things a young aquarium does. Nearly every tank I have set up has gone through it. The good news is that it usually clears itself as the tank matures, the cleanup is easy, and you do not need to panic-buy anything. Let me show you how to be sure that is what you are looking at, why it happens, and what actually shortens it.

Is it diatoms or real algae?

The quick test is to wipe a patch with your finger or a cloth. Diatoms lift off in a soft brown smear with almost no effort, leaving the glass clean underneath. Green spot algae and the tougher green films need a blade or a firm scrub; diatoms do not. If it smears away like dust and comes back a day or two later as a thin film, you have diatoms.

They are not really a plant algae in the way green algae is. Diatoms are single-celled organisms that build little glass-like shells out of silica, which is why the film has that flat, matte, mustard-brown look rather than the bright green of a proper algae problem. They settle on everything, including slow-growing plants and décor, but they sit on surfaces rather than gripping them hard. That softness is the whole reason they are so easy to deal with. For the green films that do grip, and for the wider picture, I keep a general guide to aquarium algae that covers the lot.

Why a new tank grows them

Diatoms turn up in young tanks because a young tank is the perfect home for them. A few things line up at once.

• Silicates. Diatoms feed on dissolved silica, and a new setup is full of it. It leaches out of fresh sand and substrate, and it often comes in through tap water. As the tank runs and water changes dilute it, the supply drops and the bloom runs out of food.
• An immature filter. In a tank that has not finished cycling, nutrients swing about and nothing is competing hard for them yet. Diatoms are opportunists and move into that gap. If you are not sure your tank is cycled, my guides to the nitrogen cycle and fishless cycling will tell you where you stand.
• No competition. An established tank is carpeted in invisible biofilm and, usually, growing plants, all of them using up the same nutrients and light. A bare new tank offers diatoms an open field. This is why planting heavily from day one is one of the best things you can do.

Notice what is not really on that list: hardness. People on hard water sometimes blame their tap for everything, but diatoms are a new-tank phase, not a hard-water affliction. My hard, alkaline tap grows them no worse than anyone else’s water does. Silica, not hardness, is the food here.

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The thing most people get wrong

The single biggest mistake I see is treating a diatom bloom as an emergency and throwing chemistry at it. It is a maturation phase. In most tanks it shows up somewhere between week two and week eight, peaks, and then fades on its own as the silicates deplete and the biofilter and plants take over. If you do nothing but your normal maintenance, it will very likely clear within a month or two regardless.

So the honest advice is to help it along rather than wage war on it. Here is what actually helps, roughly in order of how much difference it makes.

What actually clears it

Wipe and siphon, often. Because diatoms lift off so easily, a quick wipe of the glass and a gentle siphon over the sand at each water change physically removes most of the bloom. I do the glass with a cheap pad and vacuum the worst of it off the substrate. You are not killing them so much as carting them out before they build up.

Keep up water changes. Regular changes export silicates and nutrients and bring the bloom’s food supply down. On a new tank I stay on a steady routine; my maintenance schedule lays out what I actually do each week, and the water change calculator will size each change to your tank. If your tap is very high in silica, over time an RO cut will lower it, though I would not reach for RO just for diatoms. That is a bigger decision I cover in do you need RO water.

Get plants growing. Fast-growing or easy plants soak up the nutrients diatoms want. Even hardy, hard-water-friendly plants help once they are established. I list the ones that thrive in my water in hard water aquarium plants.

Sort out the light. Diatoms are often worse in dim or oddly timed lighting, and so is a stalled planted tank. A consistent, sensible photoperiod helps the plants win the race. I keep mine to a steady block each day rather than leaving the light on all hours; there is more on getting this right in my notes on aquarium lighting hours.

Rinse new sand before it goes in. This one is for next time. Unwashed sand dumps a lot of silica and fines into a tank and feeds the first bloom. A thorough rinse until the water runs clear takes ten minutes and saves you weeks.

The cleanup crew that earns its keep

This is where hard-water keepers have a genuine advantage, and it is my favourite fix because the animals do the work and you get to keep them afterwards.

Nerite snails are the standouts. They graze diatoms off glass, rock and leaves relentlessly, they do not breed in freshwater so they never plague the tank, and they positively need hard water to keep their shells in good condition. That makes them close to a perfect match for a tank like mine. I keep them in most setups, and a couple will keep the glass clean long after the diatom phase has passed. Full details in nerite snail care.

Otocinclus catfish are the other classic diatom grazer, and they are superb at it, mowing a film off plant leaves that a snail cannot reach. The honest caveat is that they come from softer water and can be sensitive little fish, slow to settle and easy to starve once the diatoms run out. On very hard water I lean on nerites first and treat otos as a bonus for a mature, stable, planted tank rather than a quick fix for a two-week-old one.

Amano shrimp will pick at diatoms too, though they are better value against other algae and leftover food. If you already keep them they will help; I would not buy them specifically for this. See amano shrimp care if you fancy a crew of them anyway.

A word of caution: do not add any of these to a tank that has not finished cycling just to fight the bloom. A snail or an oto dropped into an uncycled tank is exposed to the same ammonia swings as any other animal. Get the tank cycled first, then let the crew earn their keep.

When to stop worrying

If the film wipes away easily, sits on surfaces rather than gripping them, and your tank is young, it is diatoms and it is on its way out. Keep up the water changes, wipe the glass, get some plants and a nerite or two going, and let the tank mature. If instead you are fighting a bright green film that needs scraping, or a bloom that keeps roaring back in a tank that is well past six months old, that is a different problem pointing at light and nutrients, and the algae guide is the better place to start. For the common new-tank brown dust, though, patience and a snail will see you right.