Culturing Daphnia: Free Live Food, and Why Hard Water Helps

Daphnia, the little water fleas you see bobbing through pond water, are about the best live food you can grow at home, and they come with a bonus for anyone stuck on hard tap: they actively prefer it. Most of the fiddly live cultures people try, like soft-water infusoria or certain worms, feel like a chore. Daphnia are the opposite. Give them a tub of green water with plenty of minerals in it and they breed themselves into a cloud you can net out whenever the fish deserve a treat. I have run a culture on my chalky tap for years, and the hardness that fights me everywhere else is the thing that keeps it ticking.

Why daphnia and hard water get along

This is the part that surprised me. So much of fishkeeping on hard water is about damage limitation, working around the calcium rather than using it. With daphnia the calcium is the point.

Daphnia build their carapace and moult constantly, and they draw on dissolved minerals to do it. Commercial and research guidance leans the same way: keep the water on the hard, alkaline side, hold the pH roughly between 7 and 8, and in big culture tanks people even add lime to push hardness and pH up. My tap lands there on its own at around 22 dGH and alkaline, so I am starting where a lot of growers have to spend effort to arrive. If your water is anything like mine, this is one hobby project where you do nothing clever and still get the right conditions. The GH and KH explainer is worth a read if you are not sure where your own numbers sit.

Soft-water keepers can absolutely grow daphnia too, they just have a bit less buffer to play with, and a culture in low-mineral water is quicker to crash when the pH drifts.

Getting a culture started

You need three things: a container, a starter culture of daphnia, and green water to feed them.

For the container, bigger is steadier. A 20 to 40 litre tub, a spare storage box or an unused tank all work, and a wide surface area beats a deep narrow shape because daphnia want oxygen more than depth. I keep mine somewhere it gets bright light but not roasting afternoon sun. No filter. A sponge filter will suck daphnia in, and a sharp airstone chops them up, so if you aerate at all keep it to the faintest trickle of bubbles or none.

For the starter, you want live Daphnia magna or Daphnia pulex. Magna is the big one and the better feeder for cichlids; pulex is smaller and handy for fry. A single bag of either, from an aquarium shop or another hobbyist, seeds a tub easily. Tip them in and leave them be for a few days before you expect much.

Fill the tub with dechlorinated tap water, the same water you would put in a tank, and let it settle to room temperature first. On the dechlorinator question, here is where I land on that.

The book behind Lagonia

The Hard Water Aquarium

My practical guide to working with hard, alkaline tap water instead of fighting it: reading your water, choosing and setting up the tank, and keeping and breeding the fish that thrive in it, from livebearers to Rift Lake cichlids and shell-dwellers.

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Feeding the culture: green water and the alternatives

Daphnia are filter feeders. They strain tiny particles out of the water all day, so your job is to keep the water gently full of food without fouling it.

Green water is the gold standard. It is just water gone thick with single-celled algae, and it is both the food and a bit of natural filtration in one. To grow it, stand a jar or bucket of tank water in bright light, add a pinch of plant fertiliser or a squeeze of water from an algae-covered tank, and wait a few days until it turns pea-soup green. Pour that into the daphnia tub a little at a time. When the tub water clears, the daphnia have eaten the lot and it is time to feed again. If it stays clear no matter how much you add, you are overcrowded and need to harvest.

When my green water runs short, I fall back on a few cheap feeds that work:

• A pinch of active dried yeast whisked into a little warm water, added until the tub turns barely cloudy. Wait for it to clear before adding more. Overdoing yeast is the fastest way to crash a culture, so err light.
• A small amount of spirulina or chlorella powder, same method, barely cloudy.
• The green tint you siphon off the glass during a tank clean; waste algae is free daphnia food.

The rule with all of them is the same: slightly cloudy, then wait for clear. A tub you cannot see through is a tub about to go anaerobic and die.

Temperature, light and keeping it alive

Daphnia like it cooler than our tropical tanks. Somewhere around 18 to 22 degrees is the sweet spot for steady breeding. Push much past the mid-twenties and production slumps, and real heat can wipe a culture out, so a daphnia tub is better off at room temperature than next to a radiator or under tank lights that cook it. They do not need a heater at all in a normal indoor room.

Light drives the algae that feeds them, so bright is good. Long hours are fine as long as the water stays cool. A sunny windowsill that does not overheat is ideal; a south-facing sill in summer may not be.

Left alone with food and sensible temperature, daphnia reproduce without males for weeks on end, each female dropping clutch after clutch of clones. You will see the tub thicken from a few specks to a visible haze. That haze is your harvest.

Harvesting without crashing the lot

The trick with daphnia is to take some, never all. A culture crashes when you strip it bare or when waste builds up faster than you clear it.

I harvest with a fine brine shrimp net or a scrap of filter floss over a jug. Pour or scoop some culture water through it, and the daphnia collect in the net while the green water drains back into the tub. Rinse the netful in clean water and tip it into the tank. The fish will lose their minds; daphnia trigger a feeding response that dry food never quite matches, and it is genuinely good for them, a bit of fibre and the gut-stimulating wriggle that keeps digestion moving. For fish prone to constipation and bloat, that matters; I touch on the same idea in the African cichlid bloat guide.

Take maybe a third of the visible population at a sitting, no more, and let it rebuild for a day or two before the next harvest. Every week or so I siphon the muck off the bottom and top the tub up with fresh dechlorinated water and new green water. A second tub, started from the first, is cheap insurance; if one crashes you reseed it from the other and lose nothing.

Who benefits most

Daphnia suit almost everything I keep, but a few fish really earn from them.

Fry of all kinds take small daphnia once they outgrow the tiniest foods, and it bridges the gap nicely alongside baby brine shrimp. My shell-dwellers and other Tanganyikans hunt them through the water column exactly as they would in the lake. Livebearers colour up on them. Even fussy or recovering fish that ignore flake will often chase live daphnia, which makes a culture a handy thing to have running when you need to tempt a fish back to eating.

None of this costs much beyond a tub and a bit of patience, and on hard water you are playing the game on the easy setting. If you have been pouring money into soft-water projects that fight your tap, a daphnia culture is a refreshing change: the water is already right, and the fish have never eaten better.