Day 1 to Day 7: Mouth Gape Limits and First Foods
Newborn guppy fry have a mouth gape of approximately 400–500 microns, meaning any food particle larger than 0.4 mm cannot be physically ingested. Standard commercial flake food, even when crushed, typically produces particles of 0.5–2 mm — too large for reliable first-day feeding. The most appropriate first foods are infusoria (cultures of single-celled organisms including paramecia and rotifers, 50–200 microns in size), commercial fry powder labeled for "first foods" with particle size below 100 microns, or vinegar eels (Turbatrix aceti, 0.1–1.5 mm — the smaller individuals are suitable from day 1). Egg yolk paste — hard-boiled egg yolk pressed through a fine cloth into tank water — works in an emergency but rapidly degrades water quality and should not be a regular option.
Baby brine shrimp (Artemia nauplii) hatched from decapsulated cysts are the gold standard first food for guppy fry from day 1. Freshly hatched Artemia nauplii measure 400–500 microns in length — right at the upper limit of a newborn guppy's gape — and contain 60% protein and high levels of omega-3 fatty acids (EPA and DHA at 6–8% of dry weight) that support neural and fin development. Set up a hatchery 36 hours before the birth batch is expected: 1 teaspoon of Artemia cysts in 500 ml of 30 ppt salt water at 28°C with strong aeration produces approximately 200,000 nauplii, sufficient for 50 fry fed twice daily for 3 days. Harvest nauplii by shining a flashlight at the bottom of the hatchery (they are phototactic), siphon the concentrated orange cloud, and rinse through a 150-micron mesh net before adding to the fry tank to reduce salt introduction.
- ✦Feed fry 4–6 times daily in amounts consumed within 5 minutes — frequency matters more than quantity per meal at this stage.
- ✦Maintain fry tank temperature at 26–28°C; every 1°C drop below 26°C reduces metabolic rate by roughly 10% and slows growth proportionally.
- ✦Use an opaque background on the fry tank sides — fry that feel exposed spend energy on predator avoidance behavior rather than feeding.
Week 2 to Week 4: Transitioning to Microworms and Larger Particles
By the end of week 1, fry that have been fed adequately will have grown to approximately 7–8 mm body length, and their mouth gape expands to 600–800 microns. This opens the diet to microworms (Panagrellus redivivus), which average 1–1.5 mm in length and 50–70 microns in diameter — small enough to be consumed by week-2 fry in large quantities. Microworms contain 45% crude protein and 12% fat on a dry weight basis and are nutritionally comparable to Artemia nauplii for this age range. Critically, microworms sink slowly and remain active in water for several hours, making them far more practical than Artemia for multiple-daily feedings without maintaining a continuous hatchery. A single yogurt container of microworm culture maintained at 22–25°C on oatmeal substrate produces enough food for 100 fry for 1–2 weeks.
Commercial fry foods appropriate for week 2–4 include products labeled for "0–3 month fry" with a minimum 50% crude protein and maximum particle size of 250 microns. Japanese hikari first bites (50% protein, 150-micron particle) and sera micron nature (59% protein, 100-micron particle) are widely available examples. Crushed adult flake is nutritionally adequate but inconsistent in particle size — the larger particles not consumed by fry decompose within 30 minutes and create ammonia spikes dangerous to the filter-light fry tank. Pre-crushing flake through a fine mesh and measuring the crushed powder reduces this risk.
- ✦Perform 20–30% daily water changes in the fry tank using water from the established parent tank to transfer beneficial bacteria while maintaining water quality.
- ✦Add a clump of Java moss or floating hornwort to the fry tank — infusoria growing on plant surfaces supplement the diet and fry instinctively graze on plants.
- ✦Maintain 12–14 hours of light daily in the fry tank to keep fry active and feeding throughout the photoperiod.
Week 5 to Week 8: Protein Requirements and Sexual Development
Between weeks 5 and 8, guppy fry undergo rapid hormonal differentiation that determines adult coloration, tail development, and reproductive organ formation. Males develop the gonopodium (modified anal fin) and begin displaying color by week 6–7. Nutritional adequacy during this critical window has permanent effects on adult size and color intensity — fry that are nutritionally restricted during weeks 5–8 produce stunted adults with degraded color even when nutrition improves afterward. A target growth rate of 0.2–0.3 mm body length per day requires approximately 4–5% of body weight in dry food per day at 27°C — for a 1 cm fry this means 0.4–0.5 mg of food per feeding.
The diet at this stage should transition to include adult foods: high-quality flake or pellet with 45–50% crude protein as the base, supplemented with baby brine shrimp 3–4 times per week for carotenoid content, and frozen or live daphnia twice weekly for digestive enzyme stimulation. Frozen bloodworm in very small portions (a fragment 2 mm long, thawed and rinsed) once per week adds iron and heme protein that support the red hemoglobin-rich blood vessel networks developing in the tail. At week 8, males developing strong color should be separated from females to prevent early, stunting pregnancies in females and to allow selective identification of top males before they begin spending energy on courtship.
Caloric Density Comparison: Which Fry Foods Produce the Fastest Growth
Independent growth trials comparing common guppy fry foods consistently show freshly hatched Artemia nauplii produce the fastest growth in weeks 1–3, with fry reaching 1.2–1.5 cm body length by day 21. Microworm-fed fry from the same brood reach 1.0–1.3 cm by day 21 — slightly behind but adequate. Fry fed exclusively on commercial fry powder (even high-quality products) typically reach 0.9–1.1 cm by day 21. The difference is attributed to the live food's gut enzyme content — both Artemia nauplii and microworms contain active digestive enzymes that supplement the fry's own limited enzyme production in the first weeks of life, improving nutrient absorption efficiency by an estimated 20–30%.
A hybrid feeding protocol combining live food at dawn and dusk with commercial fry powder at midday feedings produces growth rates at 95% of all-live-food feeding while significantly reducing labor. This protocol: 6:00 AM — Artemia nauplii or microworms; 10:00 AM — commercial fry powder (sera micron or equivalent); 2:00 PM — commercial fry powder; 6:00 PM — Artemia or microworms; 10:00 PM — optional additional microworm feeding for heavily stocked tanks. This five-feed schedule sustains growth even in tanks stocked at the upper limit of 1 fry per 0.5 liters, which is the maximum density recommended for weeks 1–4.
- ✦Calibrate food quantity by observing the tank 5 minutes after feeding — zero uneaten food means underfed; visible food after 10 minutes means overfed.
- ✦Add a single drop of high-quality fish oil supplement (HUFA-enriched) per 20 liters of fry tank water once per week to supplement DHA in fry not receiving live Artemia.
- ✦Sort fry by size at week 3 into fast-growers and slow-growers — large fry bully small ones at the feeding area, creating a growth gap that widens through week 8.
Water Quality and Tank Size: How Environment Amplifies or Limits Feeding Gains
Growth rate in guppy fry is co-limited by nutrition and water quality — even perfect feeding produces suboptimal growth if ammonia exceeds 0.25 ppm or nitrite exceeds 0.05 ppm. A well-cycled sponge filter seeded from an established tank can handle the bioload of 30–40 fry in 10 liters with daily 20% water changes. At higher densities (50+ fry in 10 liters) the ammonia from feeding frequency outpaces the filter's nitrification capacity within days. Testing with a liquid ammonia kit (not paper strip — strips have 20–30% error rate) every other day in the first three weeks allows detection of ammonia build-up before it causes growth inhibition or immune suppression.
Tank size directly affects growth through two mechanisms: water volume dilutes metabolic waste between water changes, and swimming space allows normal territorial and exploratory behavior that keeps fry active and stimulates appetite. Fry raised in containers smaller than 5 liters consistently show slower growth than fry in 20-liter tanks even when ammonia is controlled, suggesting that confinement itself has a growth-suppressing effect, possibly mediated by cortisol stress hormones. The recommended minimum for raising a single guppy drop of 20–30 fry from birth to week 4 is a 20-liter grow-out tank, transitioning to a 40-liter tank for weeks 5–8 as the juveniles approach adult size and territorial behavior intensifies.