Why Are My Aquarium Plants Turning Brown?

Brown on a plant is a symptom, not a diagnosis. The fix for a harmless film is patience, the fix for a melting crypt is trimming, and the fix for a nutrient problem is a dosing change. Mixing them up is how a small problem turns into a bigger one. This page sorts you into the right group and points you to the deeper guide for each.

Three quick tests

Two lookalikes are worth ruling out first. Tropica notes that the black spots on the underside of Java fern leaves are its reproductive organs, not disease. And some plants are simply brown or red-brown: Tropica describes Cryptocoryne wendtii 'Mi Oya' as having red-brown leaves. If the plant is growing well and looked that way when you bought it, there may be nothing to fix.

Dusty brown film that wipes off: diatoms

What it looks like

Aquarium Co-Op describes brown diatom algae as a dusty, flour-like coating on the glass, substrate, and other surfaces, and notes that it rubs off easily. On plants it sits on top of the leaf like dust; wipe it and the leaf underneath is normal green. Tropica's algae guide groups diatoms with other thin "algal layers" that coat surfaces.

Diatoms are single-celled algae. Wikipedia describes their cell walls as made of silica, called a frustule, and their chloroplasts as yellowish-brown, which is where the color comes from.

Why it appears

Diatom causes are debated in the hobby, so treat these as associations rather than settled facts:

• New tanks. Aquarium Co-Op says diatoms are most commonly seen in newly planted tanks.
• Silicates and phosphates. Aquarium Co-Op says they are often caused by high levels of phosphates and silicates. Given their silica cell walls, a link to silicate in the water is plausible, but sources do not agree on how much it matters in a given tank.

What to do

Aquarium Co-Op calls diatoms one of the simplest algae to deal with. It says the problem tends to resolve as plants use up the excess nutrients and clean-up animals graze the film. No source we found gives a reliable timeline, so judge it by trend: a film that is shrinking week to week is going away.

In the meantime:

• Wipe the glass with a soft sponge, as Tropica suggests for algal layers, and gently rub or siphon the film from leaves and substrate during water changes.
• Keep up regular water changes. Tropica's start-up guidance is 25 to 50 percent a couple of times a week for the first three to four weeks, then about 25 percent weekly.
• Keep the light schedule modest in a new tank. Tropica suggests about 6 hours a day for the first two to three weeks before increasing to 8 to 10.

If a film is dark, slimy, sheet-like, and peels off in layers rather than wiping off as dust, it may not be diatoms. Tropica mentions blue-black coatings formed by blue-green algae, which it links mainly to excessive feeding. The algae troubleshooting guide covers how to tell them apart.

Brown, mushy, or see-through leaves: melt or rot

If the leaf itself is turning brown, soft, or translucent, you are looking at dying tissue, not a coating.

• Melt after planting. Aquarium Co-Op explains that plants grown out of water must grow new leaves adapted to life underwater, and the old leaves often die back in the process. This is common with new crypts and is usually not a sign of failure.
• Rhizome rot. Tropica warns that the rhizomes of Anubias and Java fern tend to rot if buried. A soft, dark rhizome is a different problem from leaf melt. Lift the rhizome above the substrate or attach the plant to wood or stone; see how to attach Anubias and Java fern.

Aquarium Co-Op says it does not hurt to cut off clearly melting leaves at the base, and that rotting leaves can cause nitrogen spikes or algae. Tropica also advises removing dead plant matter as soon as possible because its breakdown promotes algae. Leave the roots in place.

The plant melt guide explains how to tell normal melt from a problem, and how long recovery usually takes.

Brown spots or edges: algae or nutrients

This group is the hardest to read, so treat each explanation as a starting point.

Algae on the leaf

Firm tufts or spots that do not rub off are often algae growing on the leaf surface. Aquarium Co-Op describes black beard algae as thick, bushy clumps that are usually black or grey but sometimes reddish or brownish, growing on wood, decor, and plants. It notes that this algae is slow to establish and slow to remove. Old, slow-growing leaves, such as those on Anubias, which Tropica says can survive for years, have more time to collect algae than fast-growing ones.

The planted tank algae troubleshooting guide covers identification and causes for each type.

A nutrient explanation

Some deficiencies show up as brown marks on otherwise intact leaves:

• Potassium. Aquarium Co-Op describes pinholes that are sometimes rimmed with brown or yellow. Aquasabi describes perforated leaves or dying tissue that starts as small dark dots and grows into holes.
• Phosphate. Aquarium Co-Op describes older leaves turning yellow with soggy brown patches.

Two cautions keep this in proportion. Aquasabi recommends ruling out CO2 supply, then light, before blaming a nutrient, and notes that symptoms of different deficiencies can look very similar. Aquarium Co-Op suggests that most deficiencies are better handled by raising an all-in-one fertilizer dose than by chasing single nutrients, and says to allow at least two to three weeks to see whether a change helped. The fertilizer basics guide explains how to adjust dosing gradually.

Checks before you act

1. How old is the tank? In the first weeks, diatoms and melt are both expected.
2. Did you just add or move the plant? Recently planted points toward melt.
3. What changed recently? A new light, longer light hours, a fertilizer change, a new water source, or new livestock.
4. How long is the light on? Compare it with the start-up schedule above.
5. How much are you feeding? Tropica names overfeeding as the most common cause of algae problems.
6. Test the water. If animals are present, check ammonia and nitrite, especially if a lot of plant matter is decaying. The cycling and water testing guide explains what the readings mean, and the water test kit guide compares kits.
7. Is it one plant or many? One brown plant suggests a plant-specific cause. Brown on many plants and the glass points to the tank as a whole.

The plant problem checklist walks through these questions step by step.

What not to do

• Do not change everything at once. If you adjust light, fertilizer, and water changes in the same week, you will not know which change helped or hurt.
• Do not reach for an algaecide for diatoms. Diatoms usually fade without one. If you do use an algaecide for another algae, follow the label exactly. API's AlgaeFix label, for example, says "do not overdose" and warns against using it with freshwater crustaceans including shrimp and crabs.
• Do not spot-treat without care. Aquarium Co-Op suggests treating only part of a sensitive plant, or shortening the contact time, when using hydrogen peroxide on black beard algae.
• Do not pull up melting plants to check on them. Trim the leaves and leave the roots.
• Do not add a stack of single-nutrient supplements before ruling out light, CO2, and recent changes.
• Do not throw out a plant with brown leaves while its roots, crown, or rhizome are still firm.

A note on clean-up crews

Grazing animals can help with diatoms. Aquarium Co-Op lists otocinclus catfish, snails, and shrimp as animals that feed on them, and its otocinclus guide describes them as specialized grazers of soft green algae and brown diatoms. Tropica recommends a varied population of algae eaters, since different species prefer different algae, and it suggests introducing algae-eating snails and shrimp early in a new tank.

Before adding any of them, check three things:

• Is the tank cycled? Animals need a tank that is processing ammonia and nitrite. Confirm it with your test kit first, even if that means a few more weeks of wiping glass.
• Is there enough food? Aquarium Co-Op warns that otocinclus often arrive underfed and that their greatest weakness is not getting enough to eat. It says a couple of healthy otos is better than a group that slowly starves, and gives a temperature range of 70 to 79 °F. A diatom film that disappears can leave them hungry, so plan to supplement with algae wafers or blanched vegetables.
• Are they compatible? Aquarium Co-Op says nerite snails need peaceful tankmates that will not eat them, a tight-fitting lid because they climb to the waterline, and good water quality. Their eggs appear as small, hard white dots on hardscape but, it says, rarely hatch in fresh water. Check your existing fish, temperature, and water quality against each species before buying.

Next steps

• If leaves are melting, start with why aquarium plants melt.
• If you see tufts, threads, or spots, use the algae troubleshooting guide.
• Browse more fixes on the algae and troubleshooting hub.
• Run through the plant problem checklist before changing equipment or dosing.