How to culture green water from a starter

Green water is free-floating algae in pea-soup water — the food daphnia and moina cultures run best on, and a soft first graze for many fry. This guide is for keepers who start from a live starter (or a seeded bottle) rather than waiting on a spontaneous outdoor bloom. The DIY sun-and-patience methods live in our separate how-to-make article; here the job is keeping one reliable food tub going.

What you need

• A light-coloured bucket, tub or wide bottle (clear or white shows the green tint)
• Aged or old aquarium water — water-change water is ideal
• A sunny outdoor spot in summer, or a bright windowsill in winter
• Optional: a drop of liquid plant food or a small piece of lettuce (removed before it rots)
• Shade cloth or a loose lid if birds and dust are a problem

Seeding and growing the bloom

Tip your 250 ml starter into the tub of aged water and stand it in full sun. In a Cape summer you usually see a green tint in 3–5 days and proper soup within two weeks. Stir if you remember; don’t if you don’t — algae does not need your anxiety.

To speed things up, feed the water sparingly: a pinch of liquid plant food or a scrap of lettuce. Overdoing it turns the tub milky and smelly (bacterial bloom). If that happens, dilute with old tank water and feed less next time. The goal is green, not a drain smell.

Harvesting into the moina tub

Add green water to the daphnia/moina culture until the water takes a faint green tint — then stop. Feed again when it clears. That tint is your measuring jug: visible green means food present, clear means feed, milky-white means bacteria instead of algae (water-change the moina tub, feed less, wait).

Keep the green-water tub on a weekly harvest-and-refill rhythm: take what you need, top up with aged water, leave the rest of the bloom to rebuild. One dedicated food tub smooths out the boom-bust of casual outdoor buckets.

Cape winter and the backup rule

Outdoor blooms slow hard in a Western Cape winter. Bring a small bottle or tub onto a sunny indoor windowsill so the food line survives until spring — the same logic as splitting a moina culture. Never rely on a single outdoor tub through June–August.

Keep insecticide, copper and bleach nowhere near the culture. Shade cloth keeps birds out without killing the light. Once the tub is producing, split it: a second bottle costs almost nothing and saves a spawn when the first tub crashes.