Live food for goldfish: benefits and risks

Nothing makes a goldfish look more like the wild carp it descends from than a cloud of live daphnia. The lazy grazer becomes a hunter: darting, turning, picking off one animal at a time. Live food is a very natural thing to feed and one of the least controlled, which is why it is worth knowing what to use, where it should come from, and where to draw the line.

The live foods worth bothering with

Daphnia are small freshwater crustaceans that swim in jerks, which is why they are called water fleas. They are low in fat, high in shell, gentle on the gut and easy to culture. For goldfish they are a very good all-round live food.

Brine shrimp are hatched from cysts that keep for years in a sealed tin. Newly hatched brine shrimp are tiny and are the standard first food for fry, which feeding fry and young goldfish covers including how to hatch them; adult brine shrimp, bought live from a shop or grown on at home, suit adult goldfish. They are nutritionally modest unless enriched, but fish love them.

Blackworms are thin, dark aquatic worms sold live by the portion. They are rich, well accepted and can be kept in a shallow tray of cold water in the fridge for a couple of weeks with a daily rinse. Feed them sparingly, as a treat, for the same reasons bloodworms are best kept occasional. A related worm, tubifex, is traditionally collected from polluted mud and is best avoided unless it is farmed.

Mosquito larvae wriggle in any bucket of rainwater left standing in summer. Goldfish take them with enthusiasm, and they are free and nutritious. The obvious caution is to harvest them before they hatch, and to check whether your area discourages standing water at all.

The options compared

Food	Best for	Effort and risk
Daphnia	The all-rounder: low fat, high in shell, endlessly huntable, can be fed more freely	Low effort once a culture runs. Low risk from a culture, real from wild water.
Brine shrimp	Tempting a reluctant eater; fry food when newly hatched	Easy to hatch, more work to grow on. Low disease risk.
Blackworms	A rich treat, and excellent for conditioning	Easy if bought, but needs daily rinsing in the fridge. Buy farmed rather than collected.
Earthworms	Free, high protein, very well accepted	Almost no effort with untreated soil. Low risk from clean ground, high from treated.
Microworms and grindal worms	A cheap, endless home culture	Low effort once started, very low risk. The usual complaint is the smell.
Mosquito larvae	Free in summer, enthusiastically taken	No effort beyond a bucket. Low risk from a rain barrel, higher from open water.
Tubifex	Nothing a goldfish needs	High risk unless farmed; traditionally collected from polluted mud.
Feeder fish	Nothing at all	High risk and no benefit, for the reasons below.

Brine shrimp deserve one note from that table. They are saltwater animals, which keeps them clear of the freshwater fish parasites discussed below, so they are a low-risk live food whatever their source. Low is not none: the main bacteria in a brine shrimp hatching jar are Vibrio species.

What live food does for a goldfish

Nutrition is only part of it. A goldfish in a bare tank eats twice a day in a few seconds and has nothing to do for the rest of the day. Live food changes that. Daphnia disperse, hide in plants and have to be hunted one by one, so a meal takes an hour instead of a minute and involves the whole of the fish’s sensory equipment. Keepers who add live food regularly often describe their fish as more alert and more active generally, and it is one of the few enrichment tools that requires nothing but a net.

The risks of wild-caught food

Live food from a shop or your own culture is low risk. Live food scooped from a pond, ditch or stream is not, for two reasons.

Parasites. Many fish parasites spend part of their life cycle in crustaceans, snails or worms, and water that holds wild fish can hold their parasites too. The Merck Veterinary Manual names Cyclops and Daphnia as common intermediate hosts of the Camallanus worm, and the FAO lists fish lice and anchor worm among the parasites brought in with wild zooplankton. A netful from a fish pond can carry such stages straight into your aquarium. The risk is lower from water with no fish in it, such as a rain barrel or a fish-free garden pond, and higher the larger and wilder the source. If your fish develop white spots or go off their food after wild-caught meals, the connection is worth considering.

Pesticides and pollution. Ditches near farmland and ponds in parks may be treated for mosquitoes or weeds, and the animals in them concentrate whatever is in the water. You cannot tell by looking.

Most keepers resolve this by culturing their own daphnia and mosquito larvae from fish-free water, buying blackworms and brine shrimp, and treating anything wild as a calculated risk rather than a free lunch.

Rinsing a doubtful source

Rinsing is worth doing, and it helps to know what it leaves behind.

Rinse it. Pour the catch through a fine net and rinse thoroughly with clean dechlorinated water. That removes the pond water and any free-swimming stages of parasites, along with much of what is carried on the outside of the animals.

Holding it does not fix the rest. Keeping the catch in clean water for days does not make it safe. Parasites that use crustaceans live inside them and keep developing: the USGS gives 11 to 18 days at 29–31 °C and 49 days at 20 °C for Asian tapeworm larvae in copepods to become infective, so holding a catch can let larvae mature rather than kill them.

What that means. Rinsing removes some of the risk and does not make an unsafe source safe. A pond containing fish is still a pond containing their parasites.

Which is why the order stands: a culture you keep yourself is the lowest risk, a shop-bought portion is next, fish-free standing water after rinsing is a reasonable third, and anything netted out of water that holds wild fish is a calculated risk to take knowingly or not at all.

Feeder fish

If you are wondering about small fish as tankmates rather than food, the tankmates guide and the compatibility checker cover which species can live alongside goldfish.

Worms from the garden, and worm cultures

Two more categories, one free and one nearly so, and both routinely left out of goldfish advice.

Earthworms are excellent and rarely suggested. Carp eat them in the wild, they are high in protein, they are taken enthusiastically, and they cost nothing. Dig one from soil that has never been treated with pesticide, herbicide or artificial fertiliser, rinse it, and chop it into lengths a goldfish can swallow — an adult fancy manages a piece a centimetre or so long, and a large single-tail will take a whole small worm.

Where not to dig: a compost heap that takes cooked food waste, soil near a road, anywhere that has been treated, and a wormery. Brandling and tiger worms, the red ones used for composting, produce an unpleasant secretion when handled and are generally refused.

Microworms and grindal worms are the easy home cultures. Both live in a shallow tub of damp oat or bread paste in a warm dark cupboard, need feeding once a week, and are harvested by wiping them off the sides of the container. Microworms are tiny and suit fry; grindal worms are a few millimetres and suit adult goldfish. Both are cheap to start, almost impossible to kill, and the usual complaint is the smell if the culture is overfed.

Whiteworms are the same idea a size up, kept cooler, and are rich enough to be a treat rather than a staple.

What not to collect: garden slugs and snails, which can carry parasites and are often near slug pellets; anything from a drain or a gutter; and caterpillars or insects that may have been sprayed.

Culturing daphnia at home

A daphnia culture is among the cheapest live foods there is and takes ten minutes a week. Fill a bucket or small tub with dechlorinated water, or better, with water from an aquarium water change, and stand it in a bright window or outdoors in the shade. Let it go green with algae, or feed it a pinch of spirulina powder or a few drops of yeast dissolved in water every couple of days, just enough to tint the water. Add a starter portion of daphnia from a shop or another keeper, and within a fortnight the water will be flickering with them.

Harvest with a fine net, taking a portion every few days rather than emptying the culture, and keep a second tub going, because cultures crash without warning. Gentle aeration helps but is not essential. Outdoor tubs will also attract mosquitoes, which is either a bonus or a nuisance depending on your view.

Feeding it in practice

Live food behaves differently from anything in a tub, and a few practical points save a good deal of mess.

Net and rinse before it goes in. Pour the portion through a fine net, rinse with clean water, and tip the animals in rather than the water they were living in. Culture water carries its own bacteria and whatever the culture was fed on.

Feed where you can watch. The value of live food is partly the hunting, and the whole point is lost if the animals disappear behind the filter.

Know what uneaten food does:

• Daphnia survive in the tank and are picked off over the following hours, which is ideal. They will not foul anything.
• Brine shrimp die within a few hours in fresh water, so feed only what will be eaten fairly quickly.
• Blackworms burrow, straight into the substrate, where they survive for a long time and are occasionally rediscovered by the fish. In sand this is merely untidy; in gravel it is a reason to feed them on a dish or from a worm feeder, and the substrate page covers why a bare bottom makes this simple.
• Mosquito larvae hatch if left, which is the one genuinely important reason to feed what will be eaten.

Portion it like any other meal. Live food is food, it goes in the same budget, and a fish stuffed with daphnia is a fish that has been overfed — overfeeding goldfish covers the arithmetic. Daphnia are the exception that can be offered more freely, because they are low in fat and mostly water.

And it is a good moment to look at the fish. An hour of hunting is an hour of a fish moving normally under observation, which is more informative than a ninety-second pellet meal — signs of a healthy goldfish covers what to notice.

When live food earns its keep

Three situations where it is more than a pleasant extra.

A fish that will not eat. Live food moves, and movement triggers a feeding response in a fish that is ignoring everything static. A fish off its food for days will often take live daphnia or brine shrimp when it has refused pellets, vegetables and frozen — which matters both because the fish eats and because it shows the appetite has not gone altogether, though illness is not ruled out. Loss of appetite covers working through the causes, and feeding a sick goldfish covers the rest of that situation.

A fish in quarantine. A new arrival is unsettled, often not eating, and being watched closely — all three of which live food helps with. The one caution is source: a quarantine tank exists to keep things out, so this is not the moment for a netful of pond water.

Conditioning before a spawn. Richer live foods — blackworms in particular — are the traditional way of bringing fish into breeding condition, which is worth knowing in both directions: if you want a spawn it helps, and if you do not, heavy feeding of rich food in spring is one of the things that triggers one. Is my goldfish breeding covers the whole picture.

The live food year

Availability is seasonal, and knowing the rhythm saves both money and disappointment.

Spring. Daphnia cultures restart as the light returns, outdoor tubs come back to life, and this is the time to split a culture into two so there is a spare. Also the point at which rich live food is worth thinking about twice, for the spawning reason above.

Summer. Peak supply. Mosquito larvae appear in any standing water within days, daphnia multiply fast, and a planted pond breeds its own insects and crustaceans continuously, which is a large part of why pond fish grow as they do. The hazard is heat: a daphnia culture in direct sun crashes overnight, so keep tubs in shade.

Autumn. Cultures slow. This is the time to buy in a fresh starter if yours has weakened, and to stop relying on outdoor sources.

Winter. Indoor cultures only — microworms and grindal worms carry on in a warm cupboard regardless of the season, which is their main advantage. Outdoor fish are eating little or nothing anyway, and feeding through the seasons covers the temperature bands where most keepers stop feeding altogether.

And keep two cultures, always. Daphnia cultures crash without warning and for no visible reason, and the only reliable insurance is a second tub in a different place.

Where live food fits

Live food is a supplement in the same way frozen food is: a few meals a week, replacing a pellet meal, with a quality pellet and vegetables doing the routine work. Daphnia can be fed more freely than richer foods; blackworms and brine shrimp are best kept to once or twice a week. Pond goldfish get much of this for free, since a well-planted pond breeds its own insects and crustaceans. The general schedule is in how often to feed goldfish; the general principle is that a goldfish given something to hunt is a goldfish with a better day.