Molly Fish Tank Size: How Many Mollies in a 10 or 20 Gallon?

Reviewed by the AquariumVol Editorial Team

Mollies are one of the most popular freshwater fish — colorful, active, hardy, and available in dozens of varieties. But they're also one of the most misunderstood when it comes to tank size. The "1 inch per gallon" rule breaks down fast with mollies because they're livebearers that breed constantly, and the variety of molly you choose (short-finned vs. sailfin) changes the math entirely.

Here's the short answer: 3–4 mollies in a 10 gallon, 8–10 in a 20 gallon. But those numbers come with important conditions — read on to understand the sex ratio rules and why sailfins need more room.

Molly Varieties and Why Size Matters

Not all mollies are the same size. Before you decide how many to keep, know which type you have:

• Short-finned mollies (Common mollies): 2–3 inches. The most common pet store variety. Black mollies, dalmatian mollies, and balloon mollies fall into this category. These are the fish this guide's numbers are based on.
• Sailfin mollies: 3–5 inches, with tall dorsal fins that need horizontal swimming space. Males need at least a 29–30-gallon tank to fully display their fins. A 20-gallon works for females or a small sailfin group short-term, but it's cramped for adult males.
• Balloon mollies: A color morph with a curved spine and compressed body. Technically a short-finned molly but swims less efficiently. The same stocking numbers apply, though they need calmer water flow.

How Many Mollies in a 10-Gallon Tank?

A 10-gallon tank is on the small side for mollies, but it works if you keep short-finned varieties and stay disciplined about the sex ratio.

Safe stocking: 3–4 short-finned mollies. At this number, a well-filtered 10-gallon can handle the bioload with weekly 25% water changes. Pushing to 5 or 6 is possible with heavy filtration, but leaves no margin for error — one skipped water change and ammonia climbs fast.

Do not mix male and female mollies in a 10-gallon. The tank is too small for females to escape male harassment. Males will chase females relentlessly, causing chronic stress that weakens their immune system. In a 10-gallon, keep either all males or all females.

Sailfin mollies are not suitable for a 10-gallon tank. Their size, swimming patterns, and territorial behavior around their dorsal fin display make 10 gallons too confining even for a pair.

How Many Mollies in a 20-Gallon Tank?

A 20-gallon is where mollies really have room to thrive. You have enough space to keep a mixed-sex group and let their natural social behavior show.

Short-finned mollies: 8–10 adults with a quality filter rated for at least 40 gallons. This leaves room for a few fry to survive without instantly overstocking.

Sailfin mollies: 4–5 adults in a 20-gallon. Males may look crowded once their fins are fully developed. A 29–30-gallon is a better long-term home for sailfins, especially if you want to keep a male-female group.

Mollies Breed — Here's What to Expect

Mollies are livebearers that give birth to live, free-swimming fry every 4–6 weeks. A single female can produce 20–100 fry per batch. If you're keeping a mixed-sex group, you need a plan before the first drop arrives.

Important note: Female mollies from pet stores are almost always already pregnant. Even if you buy "only females," you'll likely see fry within 4–6 weeks of bringing them home. Stored sperm can fertilize multiple batches after a single mating.

Your fry options:

• Let nature run its course: In a planted tank with limited hiding spots, most fry get eaten by adults. Your stocking level slowly creeps up but usually stabilizes. This is the low-effort approach.
• Separate fry tank: Move pregnant females to a 5-gallon nursery tank just before birth. Grow fry to 0.5–1 inch, then rehome to a fish store for store credit.
• Males-only tank: The cleanest solution. No fry, predictable stocking level, and males still display their best colors without the stress of constant pursuit.

Molly Water Requirements: Hard Water Is Non-Negotiable

This is the care requirement that catches most new molly keepers off guard. Mollies are not soft-water fish — they come from coastal brackish and hard freshwater habitats in Central America and need mineral-rich water to stay healthy long-term.

• pH: 7.5–8.5. Mollies can tolerate neutral water (7.0) short-term, but soft acidic water below 7.0 suppresses their immune system and leads to ich, fin rot, and "molly disease" (shimmying).
• General hardness (GH): 15–30 dGH. Hard tap water is actually a plus for mollies — most municipal water in the US is hard enough.
• Temperature: 72–82°F. They tolerate a wide range but do best at 76–80°F.
• Aquarium salt (optional): Mollies famously tolerate low-end brackish conditions (1–2 teaspoons of salt per gallon). A small amount of aquarium salt can help if your tap water is soft or if fish show stress signs. Not required in hard water.
• Crushed coral or aragonite substrate: If your tap water is soft (below 8 dGH), adding crushed coral to the filter or using it as substrate raises both pH and hardness naturally.

Feeding Mollies: Don't Skip the Vegetables

Mollies are omnivores with a strong herbivore streak. In the wild, they spend a lot of time grazing on algae and plant matter. A protein-only diet leads to bloat and digestive problems over time.

Feed a quality flake or micro-pellet food as the base, and supplement 2–3 times per week with:

• Blanched zucchini or cucumber (clip to the glass, remove after 24 hours)
• Spirulina flakes or wafers
• Frozen or live baby brine shrimp (occasional protein treat)

A small amount of algae growth on tank walls and decorations is beneficial — mollies will graze on it naturally and it costs you nothing.

Best Tankmates for Mollies

Mollies are peaceful community fish that work well with other non-aggressive species that prefer similar water parameters. The best companions share the mollies' preference for hard, alkaline water:

• Guppies: Nearly identical care requirements. Both are livebearers from similar habitats. They can interbreed (guppies and mollies are different genera but same family), so keep that in mind if you want predictable offspring. See our guppy stocking guide for numbers.
• Platys and swordtails: Also livebearers with similar water needs. All four livebearer species thrive in the same water conditions. Read our platy fish stocking guide if you're mixing species.
• Corydoras catfish (4–6 in a 20-gallon): Bottom dwellers that clean up uneaten food. Most corys prefer slightly softer water, but peppered corys and sterbai corys tolerate hard water well enough to coexist with mollies.
• Nerite snails (2–4): Excellent algae controllers, won't breed in freshwater. Tollerate hard water — in fact they prefer it since calcium keeps their shells strong.
• Larger tetras: Black skirt tetras and emperor tetras tolerate neutral-to-slightly-hard water. Avoid smaller neon tetras if your pH is consistently above 7.8 — they prefer softer conditions.

Avoid: Tiger barbs (persistent fin nippers), aggressive cichlids (oscars, Jack Dempseys), goldfish (incompatible temperature and pH), and anything that requires soft acidic water (discus, most wild-caught tetras).

Signs Your Molly Tank Is Overstocked

Mollies breed fast and fry survival can sneak up on you. Watch for these early warning signs that your stocking level is creeping too high:

• Fish gasping at the surface — oxygen is depleted, usually from too high a bioload
• Ammonia or nitrite above 0 ppm — test weekly if you have a mixed-sex group
• Clamped fins — a stress response, often the first visible sign
• Increased aggression — males fighting more than usual signals competition for territory
• Fin rot appearing despite clean water — chronic stress weakens immune function

Water Testing for Molly Tanks

Mollies tolerate a wider range of water conditions than many fish, but that tolerance isn't unlimited. New tanks, growing fry populations, and skipped water changes all create risk. Test your water weekly for the first two months after stocking, then bi-weekly once parameters stabilize.

The four parameters to check: ammonia (target 0 ppm), nitrite (0 ppm), nitrate (under 40 ppm), and pH (7.5–8.5 for mollies). A liquid test kit is significantly more accurate than paper test strips — the API Freshwater Master Test Kit covers all four in one box.

Molly Fish Tank Size Cheat Sheet

• 10-gallon: 3–4 short-finned mollies, single sex only
• 20-gallon: 8–10 short-finned mollies, or 4–5 sailfins
• Sex ratio: 2:1 or 3:1 female-to-male in mixed tanks
• Water: pH 7.5–8.5, GH 15–30, temp 72–82°F
• Feed vegetables 2–3× per week (blanched zucchini, spirulina)
• Plan for fry — females from pet stores are almost always already pregnant
• Test weekly for the first two months