Dead spots
Where water barely moves, debris settles and breaks down. Cyano almost always starts there.
Nuisance growth guide
Red Slime (Cyanobacteria)
Red slime is not true algae. It is cyanobacteria, a bacteria that forms slimy mats where flow is weak and waste builds up. It is most common in reef tanks, but freshwater tanks get it too, where it is called blue-green algae. Remove it by hand, then fix the flow, light, and feeding habits behind it.
Symptoms
Is it red slime or something else?
Cyano looks and behaves differently from algae. Check these signs first.
- Slimy sheets: a dark red, maroon, or purple film on sand, rock, or glass. It can also be green, brown, or nearly black.
- Peels off easily: it lifts away in a thin sheet. Hair algae is stringy and anchored instead.
- Trapped bubbles: gas bubbles get caught in the mat under the lights, and pieces may float off late in the day.
- Starts in slow spots: corners, behind rock, and on sand where debris settles. Siphoned patches return in a day or two.
- Musty smell: a strong earthy odor, especially in freshwater tanks.
In freshwater, cyano is a blue-green or dark green slimy coat on gravel, plants, and decor. The fixes are the same, plus making sure plants are not starving for nutrients.
- Brown diatoms: a dusty brown film, common in new tanks. It wipes off like powder.
- Dinoflagellates (reef): brown, snotty strands full of bubbles, often when nitrate and phosphate test near zero. They need a different plan, so bring a photo if unsure.
- Coralline algae: a hard pink or purple crust that does not wipe off. It is healthy. Leave it.
What to test first
Find the fuel before you treat.
Cyano is a sign that something in the tank is off. These checks point to what.
Check
What you are looking for
If it is off
Nitrate and phosphate Test both on the same day.
High readings feed cyano. It can also take over when nitrate hits zero while phosphate stays high.
Feed less and improve export. In a reef, lower nutrients gradually; corals need some.
Flow Where did the mats start?
Corners and spots where debris settles mean water is not moving enough there.
Aim or add a powerhead for gentle movement across the sand.
Light schedule Hours and window light.
Long light hours, direct sun, or a recent jump in intensity.
Use a timer, block sunlight, and keep the schedule the same every day.
Mechanical filtration Socks, floss, sponges, skimmer.
Packed media holds rotting waste. A skimmer making little dark skimmate is not removing much.
Change or rinse mechanical media, then clean and adjust the skimmer.
Source water Tap or RO/DI.
Tap water can carry phosphate and nitrate. Worn RO/DI cartridges let them through.
For reef tanks, a TDS meter on RO/DI output should read at or near zero.
Likely causes
What lets red slime take hold?
Most outbreaks are two or three of these at once.
Too much food and waste
Uneaten food, frozen-food juice, and a heavy fish load add more nutrients than the tank can remove.
Dirty mechanical media
Socks, floss, and sponges left too long break trapped waste down and release it back into the water.
Weak skimming or few water changes
A poorly adjusted skimmer or skipped water changes let organics build up week after week.
Long or uneven lighting
Extra hours, window sun, or a sudden jump in intensity can tip the balance toward cyano.
Nutrients in new water
Untreated tap water or spent RO/DI cartridges add phosphate and nitrate with every top-off.
What to do today
Remove it, then starve it.
Work in order. Expect it to fade over one to three weeks, not overnight.
1
Siphon
Siphon the mats out during a water change.
Siphon the sand and use a turkey baster on rock. Removing it takes its nutrients out too.
2
Filter
Swap the mechanical media right after.
Fresh socks or floss catch loose pieces before they settle. Change them again in a few days.
3
Flow
Fix the dead spot.
Aim a powerhead at the problem area. You want gentle movement across the sand, not a sandstorm.
4
Feed
Feed smaller portions.
Feed what fish eat in a minute or two. Thaw frozen food and pour off the liquid first.
5
Light
Put the lights on a timer.
Same schedule every day, no direct sun, and no sudden jumps in hours or intensity.
6
Track
Test and log weekly.
Log nitrate and phosphate to see if your changes work. See tracking parameters.
What to avoid
Mistakes that bring it back.
Quick fixes that skip the cause usually mean the mats return.
- Reaching for a chemical treatment first. Cyano removers and antibiotic products can use up oxygen as mats die and upset filter bacteria, and cyano returns if the cause remains. If you use one, ask the store first, follow the label exactly (including any steps to remove carbon or turn off the skimmer), and add aeration.
- Stripping nutrients to zero overnight. Big drops stress corals and can open the door to dinoflagellates.
- Stirring the whole sand bed at once. It releases trapped waste. Siphon the surface in sections. See gravel vacuuming.
- Counting on a blackout alone. A few days of lights off can knock cyano back, but it stresses corals and plants, and mats return unless the cause is fixed.
- Expecting a cleanup crew to solve it. Few snails or fish eat cyano reliably.
- Scrubbing off coralline algae. Hard pink and purple crust is healthy growth, not red slime.
Helpful supplies
Store departments that help.
Pick tools that fix flow and export, not a bottle that hides the symptom.
Nitrate and phosphate tests
Show where nutrients stand. Reef tanks need a low-range phosphate test.
View testingPowerheads and wave makers
Clear dead spots and keep debris moving toward the filter.
View wave makersFilter socks, floss, and media
Fresh mechanical media catches loose cyano and waste during an outbreak.
View filtrationProtein skimmers
A well-sized, adjusted skimmer removes organics before they become nitrate and phosphate.
View protein skimmersRO/DI units and cartridges
Clean top-off and change water stops you adding nutrients every week.
View R-OSiphons and gravel vacs
The simplest cyano tool: lift mats out during every water change.
Shop maintenanceWhen to bring a water sample or call
If cyano keeps returning after two or three weeks of steady fixes, spreads onto corals, or fish breathe hard, bring a fresh water sample to the store with your tank size, livestock list, and recent changes. A clear photo helps tell cyano from dinoflagellates or diatoms.
Call (215) 269-4930 Visit the store