Red bugs are one of the few coral pests you can genuinely miss for months. They do not eat tissue in an obvious way, they do not leave bite marks, and the symptom is vague: your acros just stop looking right. Polyps stay in. Growth stalls. Color goes flat. Nothing is dying, exactly — nothing is thriving either.
What they are and how to see them
Tegastes acroporanus is a small crustacean, roughly half a millimetre, yellow-bodied with a single red dot. They are only found on Acropora, and only on the flesh, not the skeleton. At that size you are not spotting them casually across the tank — you need to look deliberately.
Two things make them visible:
- Magnification. A jeweller’s loupe, a macro phone lens, or a photo zoomed in hard. On a white or pale acro they show as tiny moving yellow specks.
- A dip cup with a white bottom. Dip an affected acro and look at the bottom of the container against the light. Red bugs come off readily and are far easier to see against white plastic than against a coral.
The most reliable early tell is behavioural: an acro that keeps its polyps retracted for days with no water chemistry explanation, while every other coral in the tank is happy. When one genus sulks and the rest do not, suspect a genus-specific pest.
Treatment
The long-standing hobby treatment is milbemycin oxime — the active ingredient in the dog heartworm medication reefers used to buy as Interceptor. It is dosed to the whole system in a series of treatments spaced roughly a week apart to catch newly hatched bugs, with heavy carbon running between doses to strip it back out.
Two things you need to know before you go that route. First, it kills other crustaceans — shrimp, crabs and copepods are all at risk, and many reefers remove what they can beforehand. Second, availability has changed over the years and dosing depends on the specific product and its concentration, so follow a current protocol for whatever you actually have rather than a decade-old forum number.
Dipping helps, and it is the right first move. A coral dip knocks adult red bugs off effectively, which makes it excellent for treating individual frags and for quarantine. What dipping does not do is clear an established infestation in a display tank, because bugs on corals you did not dip simply reseed the ones you did. Dip to protect a clean tank; dose the system to clear an infected one.
The version of this that never happens
Red bugs are exclusively an import problem. They arrive on Acropora frags, and they arrive from vendors who do not treat. If every acro entering your system gets dipped and inspected under magnification, and ideally spends two to four weeks in quarantine where you can watch it, you will not have a red bug outbreak to treat.
That is worth more than any treatment protocol, because whole-tank medication is always a compromise — you are dosing a pesticide into a closed ecosystem and hoping the collateral damage is acceptable. Not needing to is much better than doing it well.
Know what you are looking at
Half a millimetre of moving yellow could be a red bug, a harmless copepod, or an amphipod. Most of what moves in a reef tank is beneficial, and treating the whole system for a pest you do not have is a bad trade.
Our Coral Pest ID & Treatment Guide is a 29-page photo reference to the pests and the look-alikes — red bugs, flatworms, nudibranchs, vermetids, aiptasia, asterina and more — with the treatment approach for each. Instant PDF download.
For the inspection and dip routine that keeps them out in the first place, see The Complete Coral Dipping Guide.