Green, pea-soup-colored aquarium water is a free-floating algae bloom, almost always caused by too much light — too many hours, too much intensity, or direct sunlight — combined with excess nutrients from overfeeding or infrequent water changes. Cutting back light exposure and treating the current bloom directly are both usually needed to fully clear it.

How do I get rid of green water in my fish tank?

Three things need to happen together: reduce the light feeding the algae, reduce the nutrients it's consuming, and treat the bloom that's already there. Skipping any one of the three tends to mean the green tint comes right back within a couple of weeks.

What's actually causing it?

  • Too much light — more than 8-10 hours a day, or a tank that's simply too bright for its bioload, feeds free-floating algae directly.
  • Direct sunlight hitting the tank is one of the most commonly overlooked triggers — even a few hours of window light can be enough.
  • Excess nutrients from overfeeding, overstocking, or going too long between water changes give algae more to consume.

Step-by-step fix

  1. Cut back your lighting schedule to 6-8 hours a day — a plug-in timer keeps this consistent — and move the tank out of direct sunlight if that's a factor.
  2. Reduce feeding slightly and do a partial water change to bring nutrient levels down.
  3. Treat with an algae control product to clear the current bloom faster than light/nutrient changes alone would.
  4. If you want a longer-term structural fix, add fast-growing live plants — they compete directly with algae for the same nutrients and tend to win once established.

Will a UV sterilizer fix green water?

A UV sterilizer can be very effective against a free-floating algae bloom specifically, since it kills algae cells as water passes through it — more so than it helps with algae growing directly on surfaces (see our algae on plant leaves guide for that case, which needs a different approach). It's a reasonable investment if green water is a recurring problem in your tank rather than a one-off.

How long until it clears?

With light reduced and an algae treatment applied, most green water blooms noticeably improve within a few days and fully clear within 1-2 weeks. If it hasn't budged after two weeks, double-check that a sunlight source isn't still hitting the tank — this is the single most common reason a green water fix doesn't stick.

Not sure green water is what you're dealing with? Run the 2-minute water diagnosis to confirm, and see the full green water guide for step-by-step fixes.