Problems and help

Reef tank troubleshooting: find your problem

Start from what you can see. Each page says what to check first, the likely causes in order, what to do, and when to get help. Something dying right now? Go straight to the emergency playbooks.

Last checked 21 October 2026. How we check

What are you seeing?

• EmergencyPower cut

  Get water moving first, then manage temperature, then restart pumps, heaters and the skimmer in a safe order.

• EmergencyHeater stuck on (tank too hot)

  Unplug the heater, turn the lights off, cool gently with a fan and sealed ice, and keep oxygen up.

• EmergencyOverdose

  Doser stuck on or a dose added twice: stop dosing, test, and let it come back down slowly rather than forcing it.

• EmergencyLeak or flood

  Make the electrics safe first, find whether the leak is in the sump or the display, lower the water below the leak, then save the livestock.

• EmergencyTop-off (ATO) failure

  Top-off stuck on (salinity falling) or empty (salinity rising): what to unplug and how to bring salinity back slowly.

• EmergencyThings are dying (tank crash)

  Several animals dying at once: the first hour checks, in order, before you buy anything.

• Water and chemistryCloudy water

  Milky haze in a new tank, a white snowstorm after dosing, or sand dust after setup: how to tell them apart and fix each one.

• Algae and slimeAlgae by type

  Identify hair algae, bubble algae, green film and brown dust by look, then fix the cause and choose grazers that will actually eat it.

• Algae and slimeCyanobacteria (red slime)

  Red, purple or dark green sheets on sand and rock: how to confirm it is cyanobacteria, why it grows and the order to fix it.

• Algae and slimeDinoflagellates

  Brown, snotty strands with bubbles that disappear after lights out: how to confirm dinoflagellates and the first things to change.

• Algae and slimeDiatoms (brown dust)

  Brown dusty film on sand and glass in the first weeks is usually diatoms: what feeds them, how long they last and what helps.

• CoralsCorals not opening

  Closed or shrunken corals: the order to check salinity, temperature, pests, light, flow, recent changes and alkalinity.

• Water and chemistryAlkalinity swings

  Alkalinity jumping between tests: how dose timing, kalkwasser, low magnesium, doser drift and test error cause swings.

• FishFish scratching (flashing)

  Fish flashing or rubbing on rock and sand: whether it is a parasite or water, how fast to act, and when to set up a treatment tank.

• FishWhite spots on fish

  Salt-grain spots or fine dust on a fish: how to tell marine whitespot (ich) from velvet, and why velvet needs action the same day.

• FishFish not eating

  A new or settled fish refusing food: stress, bullying, disease, water quality or the wrong food, and what to offer first.

How do I stop problems before they start?

Many of the losses keepers describe follow a failed heater or top-off, a fish that skipped quarantine, or a holiday. These guides deal with each.

• GuideHolidays and tank sitters

  What to set up weeks before, a sitter care card to print, and a daily check sheet.

• GuideQuarantine for fish and corals

  Options a beginner will actually use, coral dips and how long to wait.

• GuidePests and hitchhikers

  Which hitchhikers are harmless, which pests to remove, and what each coral dip kills.

• GuideUK reef keeping guide

  Running costs at the current energy cap, buying and delivery, medicines, CITES and gallons.

• GuideGlossary

  Plain definitions of reef-keeping terms and abbreviations.