Diatom Bloom in New Planted Tanks: Why It Happens and How to Wait It Out

# Diatom Bloom in New Planted Tanks: Why It Happens and How to Wait It Out

You set up your planted tank on Saturday. By Wednesday there’s a fine brown film on the glass, the sand, the driftwood, and the leaves of your new plants. You’re scrubbing it off every morning and it comes back every evening. Your tank looks like someone dusted it with cocoa powder.

That’s a diatom bloom, and if your tank is under eight weeks old, it’s close to inevitable. Here’s what’s actually happening and what you can do about it.

## What diatoms actually are

Diatoms are single-celled algae with silica-based cell walls. Unlike green algae or cyanobacteria, they don’t need strong light to thrive – they thrive when a new tank has excess silicate and silicic acid in the water, which comes from the substrate, the glass, the silicone sealant, and some tap water sources. New Aquasoil and similar plant substrates leach ammonia and silicates heavily during the first four to six weeks, which is the same window diatoms appear.

The brown film you’re seeing is millions of these silica shells. It wipes off glass easily with a razor blade or magnet cleaner. It coats the substrate, the hardscape, and unfortunately it also coats plant leaves, which blocks light and slows growth right when your plants are trying to establish.

## Why water tests won’t help you here

If you test your water during a diatom bloom, you’ll see ammonia and nitrite spikes from the substrate, possibly raised phosphates, and silicates you can’t easily test for (most hobbyist kits don’t include silicate tests). The ammonia is the trigger – diatoms are opportunistic and bloom in water that hasn’t established a nitrifying bacterial population yet.

This is the same process as a fishless cycle, just with plants and algae competing for the same nutrients. The bacteria that convert ammonia to nitrite to nitrate take four to eight weeks to establish in a new tank at 78°F. Your plants are also trying to establish root systems during this window and can’t outcompete the diatoms for nutrients yet.

## What actually works

### Wait

The bloom peaks between week two and week four, then declines as the bacterial population matures and the substrate stops leaching. Most tanks clear on their own between week six and week eight. If you set up with established filter media from a mature tank, the bloom is shorter – sometimes two weeks – because the bacterial population is already partially established.

### Reduce the photoperiod

Eight hours of light is enough for most plants during establishment. Running twelve hours doesn’t grow plants faster – it just gives algae more energy. Set a timer and keep it consistent. The spectrum matters too: warm white LEDs around 6500K grow plants adequately without the deep reds and blues that algae prefer.

### Stop adding fertilizers

New substrate leaches nitrogen and phosphorus for weeks. Adding NPK fertilizers on top of that gives diatoms exactly what they need. Wait until the ammonia reads zero and your plants show new growth before starting a dosing schedule – usually week six at the earliest.

### Mechanical removal

Wipe the glass daily with a magnet cleaner or acrylic scraper. Siphon the substrate during water changes – not deep gravel vacuuming, just the surface detritus. Clean filter intake sponges weekly in old tank water, not tap water. None of this stops the bloom but it keeps the tank looking presentable while the biological filter matures.

### Water changes

Thirty percent weekly is sufficient. Larger changes don’t speed up the cycle because the bacteria live on surfaces, not in the water column. What matters is keeping ammonia below 0.5 ppm so your plants don’t burn. If ammonia spikes above 1 ppm, do a 50% change.

## What doesn’t work and makes things worse

### Adding algae eaters

Otocinclus, nerite snails, and Amano shrimp are the usual recommendations. They won’t solve a diatom bloom – the bloom is caused by water chemistry, not a lack of grazers. Adding livestock to an uncycled tank with ammonia spikes risks killing the animals. If you want cleanup crew for a planted tank, add them after the cycle completes and ammonia and nitrite read zero for two straight weeks.

### UV sterilizers and algaecides

UV sterilizers kill waterborne algae and pathogens but do nothing for diatoms on surfaces. Algaecides containing copper will kill your shrimp and snails before they kill diatoms. The chemical route is a dead end here.

### Bleach dips and tank resets

Tearing down a tank that’s cycling to “start over” just resets the clock. The diatoms come back because the new setup has the same silicate source and the same lack of bacterial competition. Patience is cheaper and more effective.

## When the bloom doesn’t clear

If you’re past week eight and the bloom is still going strong, the issue is probably one of three things:

1. **Your light spectrum is wrong.** Some “full spectrum” plant lights emit heavily in the 440nm blue range, which feeds algae more than plants. A 6500K white LED grows plants adequately without the algae candy.

2. **Your biofilter is undersized.** A filter rated for a 20 gallon on a 20 gallon tank has no margin. Over-sizing the filter by 50% gives you bacterial capacity to spare and the flow rate keeps detritus from settling.

3. **You have phosphate from your source water.** Test your tap water. Some municipal supplies run 1-2 ppm phosphate. If yours does, a phosphate-removing resin in your filter is the actual fix, not water changes.

## The realistic timeline

Week 1-2: Substrate leaches ammonia. Diatoms appear on glass and hardscape. Plants show melt as they transition from emersed to submerged growth.

Week 3-4: Diatom bloom peaks. Ammonia and nitrite are detectable. Plants show new submersed growth if the light is adequate.

Week 5-6: Ammonia drops to zero. Nitrite spikes, then drops. Diatoms start receding. Green algae may appear on the glass in small amounts.

Week 7-8: Nitrate is the only detectable compound. Diatoms are gone or limited to bright spots on the glass. Plants are growing visibly.

Week 9+: The tank is cycled. You can start a regular fertilizing schedule, add cleanup crew, and begin normal maintenance.

This timeline assumes 78°F, planted with fast-growing stem plants, and a properly sized filter. Bare tanks or tanks with slow-growing plants only cycle slower.

## What to tell people who ask

If someone sees your tank during the bloom and asks what’s wrong, the answer is “it’s cycling.” Every new planted tank goes through this. The tanks you see on Instagram were either cycled for months before the photo or they’re going through the same bloom behind the camera. There’s no product that skips this step – the bacterial population has to grow, and it grows at its own pace.

## Sources and further reading

The nitrogen cycle in aquariums is documented in every introductory aquatic ecology textbook. The relationship between silicate loading and diatom blooms is covered in [this review of freshwater algal ecology](https://tanninlab.com/betta-fish-tank-setup/) from the University of Florida’s aquatic plant management program. The [planted tank substrate guide](https://tanninlab.com/planted-tanks/) on this site covers what Aquasoil leaches and for how long.