Algae
Black or dark red tufts are growing on my plants and decor
Dark tufts that feel tough and grip hard are what most keepers call black beard algae. Test nitrate and change water on your usual schedule first, then look at where the tufts have settled — they take the slowest-moving, longest-lived surfaces, and that is the diagnosis. Improving flow, cutting affected material out and steadying the tank does more than anything you can pour in.
Act this week
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- pH and KH — acidity of the water, plus the carbonate buffer that resists changing it
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| Dead spots where water barely moves | The tufts are concentrated behind decor, in corners, on the filter intake and on the substrate edge farthest from the outflow. Drop a pinch of food and watch where it settles rather than travels. | Six to eight weeks before new growth stops appearing |
| Old, slow-growing plant leaves providing a foothold | The worst growth is on the oldest leaves of slow plants — anubias, java fern, buce — and on nothing that grew in the last month. | Immediate removal, with regrowth judged over four to six weeks |
| Nutrient accumulation from an overdue schedule | Nitrate reads above 40 ppm, the substrate releases brown clouds when disturbed, and the last full water change was more than two weeks ago. | Four to six weeks of consistent maintenance |
| Conditions that keep shifting week to week | KH is low, pH moves between tests, and carbon dioxide or fertilizer dosing has been irregular. This alga does well precisely where the tank is unsettled. | Two to three months to change the tank's character |
| New material brought in on a plant or decoration | It appeared on one new plant first and spread outward from there over several weeks. | Weeks, depending on how far it spread before you noticed |
Dead spots where water barely moves
How to confirmThe tufts are concentrated behind decor, in corners, on the filter intake and on the substrate edge farthest from the outflow. Drop a pinch of food and watch where it settles rather than travels.
What to doReposition the outflow or add a small powerhead so the whole tank turns over. Clear the intake strainer at every water change. Flow will not kill existing growth but it stops new colonies establishing.
Time to resolve: Six to eight weeks before new growth stops appearing
Old, slow-growing plant leaves providing a foothold
How to confirmThe worst growth is on the oldest leaves of slow plants — anubias, java fern, buce — and on nothing that grew in the last month.
What to doCut the affected leaves off at the base and remove them from the tank rather than trying to clean them in place. Fast-growing plants are rarely affected because they outgrow it.
Time to resolve: Immediate removal, with regrowth judged over four to six weeks
Nutrient accumulation from an overdue schedule
How to confirmNitrate reads above 40 ppm, the substrate releases brown clouds when disturbed, and the last full water change was more than two weeks ago.
What to doReturn to a weekly 25 to 30% change with substrate vacuuming. This does not remove the algae directly; it removes the surplus it is living on.
Time to resolve: Four to six weeks of consistent maintenance
Conditions that keep shifting week to week
How to confirmKH is low, pH moves between tests, and carbon dioxide or fertilizer dosing has been irregular. This alga does well precisely where the tank is unsettled.
What to doSteady everything you control before you attack the algae: a fixed light period, a fixed water change day, a stable KH. Consistency is the actual treatment here and it is unexciting.
Time to resolve: Two to three months to change the tank's character
New material brought in on a plant or decoration
How to confirmIt appeared on one new plant first and spread outward from there over several weeks.
What to doRemove the affected material. Quarantine new plants in a separate container for a couple of weeks before they join the tank, and inspect anything secondhand carefully.
Time to resolve: Weeks, depending on how far it spread before you noticed
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not dose bleach or hydrogen peroxide into a tank with fish in it because a forum post said so. Whatever the intended target, it goes into the water the animals breathe.
- Do not scrub affected leaves and leave them attached. Cut and remove.
- Do not turn the flow up so far that fish struggle to hold position. The goal is no dead spots, not a current.
- Do not expect a fast result. This one is slow to arrive and slow to leave.
Keeping it from coming back
This alga rewards inconsistency, so the prevention is a boringly regular tank: the same light hours every day, the same water change day every week, a KH that does not sag, and flow arranged so nothing sits still. Quarantine new plants before they go in, keep the filter intake clear so the turnover you paid for is the turnover you get, and trim old leaves before they become the oldest surface in the tank.
Reference for this page
Equipment involved
- Filtersturning water over and housing the bacteria
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
- Test Kitsknowing the number instead of guessing it
Related problems
Watch it
Act this week
Act this week
Published sources
- US EPA — Nutrient Pollution
- USGS Water Science School — Hardness of Water
- NOAA National Ocean Service — Harmful Algal Blooms
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.