Why Did My Tub Turn Green?

What green water actually is

Not dirt, not disease, and not the slimy algae on the walls. Green water is billions of free-floating, single-celled algae, phytoplankton, blooming because your water is offering them what every plant wants: light and nutrients with no competition. It is most common in exactly the setups that are otherwise doing well, a new tub in good sun, generously fed, before the plants have caught up.

The same pond, two moods. Green water is a feature until it is a nuisance. My illustration.

Why it happened to yours

Some combination of: the container is new, so nothing else is consuming the nutrients yet; it gets strong direct sun; feeding is heavier than the fish need, and uneaten food converts straight into algae fuel; and there are not enough live plants competing for the same light and nitrogen. A green tub is almost a diagnostic printout of those four dials.

The fix that lasts: outcompete it

Floating plants are the treatment. They take the two things the bloom runs on, shading the light and drinking the nutrients, and a healthy raft of floaters starves a bloom out over a few weeks in a way that keeps it from returning. Submerged plants pull from the same account. Pair that with feeding less, which is nearly always part of the answer, and most green tubs clear on their own schedule and stay clear.

The fixes that disappoint

Water changes clear it for a day or two, then it rebounds, because you removed algae but not the light and nutrients growing it. Fine as a cosmetic assist, not a cure. Algaecides I would skip entirely in a stocked container: a bloom killed all at once decays all at once, and that decay can pull the oxygen out from under your fish. Blackout methods work but stress your plants too. For a true pond-scale problem, a UV clarifier is the equipment answer, though a medaka tub rarely justifies one.

When you might keep it

A greenhouse secret worth knowing: green water is first-class fry food, mostly indirectly. The algae feed the rotifers and paramecium that newly hatched medaka fry actually eat, and breeders deliberately culture green water for their fry tubs. If your green container happens to be where eggs are hatching, you are accidentally running the classic setup. My fry guide covers the rest.

What it is not

String algae on the walls is a different organism with different fixes, and a snail crew grazes it well. Cloudy white or grey water is usually a bacterial bloom in a new setup, and it clears itself. Foul-smelling or murky brown water points to rotting debris and does deserve a cleanup, while clear tea-colored water is usually just tannins from leaves or soil and is harmless. Green means growth. Of the ways a tub can discolour, it is the friendliest.

Common questions

Is green water harmful to medaka?

No. Green water is a bloom of suspended single-celled algae, and it is close to harmless for medaka. Fry keepers deliberately culture it as food for newly hatched fry. The main costs are that you cannot see your fish and that a very dense bloom can drop oxygen overnight and push pH up in the afternoon, and one that dies suddenly can crash oxygen, so keep the surface moving in hot weather.

What causes green water in a fish tub?

Light and nutrients with nothing competing for them: a new setup, strong direct sun, generous feeding, and too few live plants. The bloom is consuming what nothing else in the container is using yet.

How do I get rid of green water?

Outcompete it. Floating plants shade the light and consume the nutrients the bloom runs on, and paired with lighter feeding, most containers clear within a few weeks and stay clear. Water changes alone rebound within days, and algaecides are risky in stocked containers because a mass die-off can crash the oxygen.

Is green water good for medaka fry?

Yes, mostly indirectly. Green water grows the rotifers and paramecium that newly hatched fry actually eat, and breeders culture it on purpose for fry containers. A green tub where eggs are hatching is the classic setup by accident.