UV Sterilizers
A UV sterilizer does one job extremely well and several jobs not at all. Understanding which is which saves you either a wasted purchase or an outbreak you could have ended in three days.
What UV actually kills
UV light kills organisms that pass through the sterilizer chamber, in the water column, while the lamp is on. That covers free-floating single-celled algae — green water — and the free-swimming stages of some parasites and bacteria.
It does not touch anything attached to a surface. A UV unit will not remove black beard algae from your wood, green spot from your glass, ich cysts from a fish, or anything living in the substrate or filter media. If the problem is not floating past the lamp, UV is irrelevant to it.
Green water: the case where it just works
A green water bloom is the one problem where UV is close to a guaranteed fix. Nothing else — not blackouts, not water changes, not flocculants — is as reliable. Plumb the unit in, run it continuously, and expect visibly clearer water in two to three days and full clarity in about five. Fix the underlying cause too (excess light, excess nutrients, an uncycled tank), or it comes back the week you switch the lamp off.
Flow rate is the setting people get wrong
Kill rate depends on how long the water sits under the lamp. Manufacturers publish two very different flow ratings for the same unit: a fast rate for algae control and a much slower one for bacteria and parasites. If you are chasing a parasite, you need to throttle flow down to the slower figure — running at the algae rate simply will not do it. Check the spec sheet rather than assuming more flow is better.
Replace the lamp on a schedule
This is the most common failure and it is invisible. UV lamps lose output long before they stop lighting up. A bulb that still glows blue at eighteen months may be putting out a fraction of its rated germicidal intensity. Replace on the manufacturer's schedule — typically every six to twelve months of running time — regardless of whether it looks fine.
What it will not harm
Your biological filter is safe. Nitrifying bacteria live on filter media and surfaces, not drifting in the water column, so UV does not touch the colony. Fish, shrimp and plants are unaffected as long as the lamp is enclosed in the unit.
One real caveat for planted tanks: UV can break down chelated iron and some trace elements in the water column. In practice this means dosing slightly more iron, or dosing right after lights-out with the sterilizer briefly off. It is a nuisance, not a reason to skip UV.
Twinstar units are a different technology
Worth being clear about, since they sit in this category. Twinstar devices are not UV sterilizers — they work by electrolysis, generating short-lived oxidizing compounds in the water rather than shining a lamp through it. They are aimed at suppressing algae in the early weeks of a new planted tank, when a fresh scape is most vulnerable, and at reducing pathogen load in shrimp tanks. Different mechanism, different use case. If green water is your problem, a UV unit is the right buy; if you are setting up a new high-tech scape and want to get through the establishment period cleanly, the Twinstar is what people use.
Standalone or pump-integrated?
Inline canister units and sump-mounted units give you control over flow rate, which matters for parasite work. Submersible pump-and-UV combination units are simpler and cheaper and fine for algae control in a tank with no sump. See pumps for the combination units.
Tell us the problem first
Half the people who come in asking for a UV sterilizer have a problem UV cannot solve, and we would rather say so than sell you one. Bring a photo of the tank and a water sample — both stores test for free — and we will tell you whether this is the right tool or whether you are looking at a lighting, nutrient or flow issue wearing a different hat.