Reef Tank Troubleshooting: Algae, Pests and Swings

Something looks wrong, and the first instinct is to buy a bottle. Reef tank troubleshooting works better in the other direction: name the symptom precisely, shortlist the causes that could produce it, run the one test that separates them, then change a single thing and wait. Most of the calls we take from Chicago homes and offices are not mysteries. They are a nutrient imbalance, a hitchhiker that arrived on a frag plug, a parameter drifting for a month, or equipment quietly doing something different than it did in June. This is the tree we work through, in the order we work through it.

Reef tank troubleshooting starts with the symptom, not the product

Write down what you see, not what you think it means. "Rust-brown dust on the sand and lower glass, started nine days ago" is a symptom. "Algae" is a category, and categories send people to the wrong shelf. Note the date it appeared and what changed in the fortnight before, because most reef problems trail their cause by seven to fourteen days. A new light schedule, heavier feedings, a house guest who fed the tank, a skimmer you cleaned and never retuned: any of those can be the thing.

Then work the same four steps every time, in the same order:

1. Record the symptom as a dated observation, not as a diagnosis.

2. List the two or three causes that could produce exactly that symptom.

3. Run the single test that separates them, not a full panel.

4. Make one change, log it, and give the tank a week or two before judging it.

Your test kits are part of the diagnosis, so treat them as suspects too. Reagents age, and a kit stored above a warm sump drifts faster than one in a cabinet. Test at the same hour, since alkalinity and pH both move on a daily rhythm, and calibrate a refractometer with 35 ppt standard fluid rather than RO water. If a reading looks alarming, confirm it with a second kit before acting. Plenty of the emergencies we get called into are a tired reagent and a panicked correction, one of the most common reef tank mistakes beginners make.

Algae tells you which input is out of balance

Brown dust on sand, rock and glass in the first two months is almost always diatoms, fed by silicate from source water and freshly leaching dry rock. The test is a TDS meter on your RO/DI output, which should read 0 ppm, plus a silicate kit if the meter is clean. The fix is source water and patience: replace exhausted DI resin, keep up small water changes, and let trochus and cerith snails graze the surfaces. A young tank that browns and then clears was never sick.

Green hair algae is a different message. Long strands anchored to rock and growing back within days of a scrub usually mean phosphate above roughly 0.1 ppm, with light and organics to match. Confirm with a low-range phosphate checker and a nitrate test on the same sample, then export before you dose: pulling by hand, changing the filter sock and feeding less are safer first moves than a reactor. Bring nutrients down slowly, because crashing them starves corals and trades hair algae for something worse.

Red or purple slime that peels off in sheets is cyanobacteria, and it colonizes dead spots, so check flow before chemistry. Brown stringy snot with trapped bubbles that recedes overnight and returns by afternoon is more likely dinoflagellates, which thrive when nitrate and phosphate both read zero. Those two get opposite treatment: cyano wants more flow and less organic load, while dinos usually need nitrate lifted toward 5 to 10 ppm and phosphate toward 0.03 to 0.1 ppm, often with a UV sterilizer. Our guide to keeping algae from taking hold covers the export system that stops the cycle repeating.

Pests hide by day and give themselves away at night

Aiptasia is the pest people notice first: small brown anemones with long tapered tentacles that snap into the rock as you approach. Majano anemones are squatter, greener and just as invasive. Scraping either one spreads it, since fragments settle and regrow. Injecting the base with kalkwasser paste or a purpose-made aiptasia product works on a handful of polyps, and Berghia nudibranchs are the better answer for a covered rock. Peppermint shrimp sometimes eat aiptasia and sometimes ignore it, so treat them as a bonus rather than a plan.

Coral-specific pests are quieter. Acropora-eating flatworms leave bare bite marks at the base of branches and lay egg clutches on exposed skeleton. Montipora-eating nudibranchs show as white patches spreading across a plate, with tiny white slugs tucked underneath. Red bugs appear as yellow specks on smooth-skinned acros whose polyps have stayed shut for weeks. The confirming test is the same for all three: baste the colony hard in a container of tank water, or run a coral dip, then study what falls out against a white background.

Treatment is a cycle, not a single event, because dips kill adults and not eggs. Dip affected colonies every five to seven days for three or four rounds, picking egg clutches off the skeleton with tweezers in between. Quarantine every new coral for four to six weeks with a dip on arrival, and cut frags off plastic plugs, a favorite hiding place. Not every crawling thing is guilty: bristle worms, amphipods and micro-brittle stars are detritivores earning their keep, and treating for them does real harm.

Swings do more harm than an imperfect number

Stability beats perfection, and it helps to know what stable looks like. Typical targets are salinity 1.024 to 1.026 specific gravity, temperature between 76 and 80 degrees Fahrenheit holding within about a degree, alkalinity 8 to 11 dKH, calcium 400 to 450 ppm, magnesium 1,250 to 1,400 ppm, nitrate around 2 to 10 ppm and phosphate 0.03 to 0.1 ppm. A reef held steady at 7.8 dKH does better than one bouncing between 8 and 11, so read your numbers as a month-long trend rather than a score for today.

The symptom that points here is tissue receding from the base of stony corals, or polyps that stayed shut after something you did. If it followed a large water change, a dosing correction or a new bucket of salt, alkalinity is the first suspect: test at the same hour on two consecutive days and compare the swing rather than reacting to one reading. Salinity is the second suspect, and it climbs quietly through a Chicago winter when forced-air heat pushes evaporation past what your top-off can cover.

Before you dose anything, check the hardware, because equipment causes more swings than chemistry does. A stuck top-off valve dumps fresh water and drops salinity fast. A heater that fails on cooks a small tank overnight, and one that fails off gives a slow chill that shows first as closed polyps. A skimmer overflowing after a cleaning pulls the sump down, a clogged return kills flow on one side of the display, and a heavy kalkwasser drip lifts pH past 8.5. Look, listen and touch the equipment first, then test.

Fish behavior is the fastest reef tank troubleshooting signal you have

Start by asking whether one fish is affected or all of them. Every fish hanging near the surface breathing hard, worst in the early morning, points to oxygen and temperature rather than disease. Warm water holds less oxygen, and a stalled skimmer or dead powerhead removes the surface agitation that drives gas exchange. Aim a powerhead at the surface, get the skimmer running, bring the temperature slowly toward the upper 70s, and hold off on medication until breathing normalizes.

One fish behaving strangely is a different branch. White specks the size of salt grains that come and go on a three to seven day rhythm suggest marine ich, while fine gold dust with fast gill movement and decline over one to three days is far more likely velvet, the emergency of the two. Clownfish shedding sheets of white mucus point toward brooklynella. Flashing with no visible spots often means flukes. Look at the fish side-on with a flashlight, count gill beats, and watch whether it still eats.

The fix belongs in a hospital tank, not in your display. Copper and most medications are lethal to snails, shrimp and corals, and they bind into rock and sand, so a display dosed with copper is compromised for good. Move the affected fish to bare-bottom quarantine for treatment, and leave the display fishless for roughly 76 days if you want the parasite to die out. It is slow, unglamorous work, and it is why so many reef keepers hand the routine checks to somebody else.

Key takeaways

• Reef tank troubleshooting is a loop: describe the symptom, shortlist causes, run one confirming test, change one thing, then wait a week or two.

• Algae type is diagnostic. Diatoms point at source water, hair algae at phosphate and export, cyano at flow, dinoflagellates at nutrients stripped to zero.

• Pests need a dip cycle rather than a single treatment, because eggs survive the first round; quarantine new corals for four to six weeks.

• A steady parameter beats an ideal one, and most swings trace back to equipment rather than to chemistry.

• All fish breathing hard at once is an oxygen or temperature problem; one fish with spots or heavy gills is a disease problem for a hospital tank.

Frequently asked questions

Why did my corals close up right after a water change?

Usually a parameter gap between the tank and the new water. Salt mixed cold, mixed for less than a day, or from a different bucket can differ in alkalinity by several dKH. Match temperature and salinity, mix and circulate the batch overnight, test the mixed water before it goes in, and keep single changes to around ten to fifteen percent.

How long should I wait before trying a second fix?

Give any change seven to fourteen days unless livestock is actively dying. Biological systems respond slowly, and stacking a second treatment on top of the first makes it impossible to know which one worked or which one caused the next problem. Log the date and the change, then judge the result against your notes rather than against memory.

Is a three-day blackout safe for corals?

Most healthy corals tolerate three dark days, and it can knock back cyanobacteria when paired with better flow and less organic waste. It rarely solves dinoflagellates on its own, and stressed or freshly shipped colonies handle it poorly. Keep flow and heat running, skip feeding heavily during it, and restore light gradually rather than at full intensity.

Work with Marine Concepts Inc

If a problem has outlasted two rounds of fixes, a second pair of eyes on the system usually finds it faster than another product will. We can take over the testing, water changes and equipment checks on a regular aquarium maintenance schedule anywhere in Chicago and the suburbs, and test kits, dips, salt and replacement parts are on the shelf for same-day pickup at Marine Concept Supply in Wheeling, IL.