Molly vs Guppy

Quick comparison table

Category	Molly	Guppy
Type	Freshwater livebearer	Freshwater livebearer
Difficulty	Beginner (if water is hard)	Beginner
Temperament	Peaceful, active	Peaceful, active
Adult size	~3–4.5 inches	~1–2.5 inches
Minimum tank	20 gallons	5 gallons (better 10–20)
Recommended tank	30 gallons	10–20 gallons
Temperature	75–82 F	72–82 F
pH	7.5–8.5 (hard, alkaline)	6.8–7.8 (flexible)
Hardness	Hard preferred	Soft to moderate OK
Breeding	Prolific	Extremely prolific
Community role	Larger midwater presence	Smaller, flashier school/group

Profiles: Molly · Guppy

Overview

Mollies and guppies are both livebearers sold as “easy.” They are easy only when the tank matches them. Guppies stay smaller and adapt to more water types. Mollies grow larger, prefer hard alkaline water, and need more swimming room. Put mollies in a soft, tiny community and you get shimmies, stress, and mystery deaths.

Size and tank requirements

Guppies work in 10–20 gallon planted tanks as a group. A 5-gallon is a bare minimum for a tiny all-male setup — not a mixed breeding colony.

Mollies want 20–30+ gallons. Adults over 3 inches with constant grazing activity load a filter harder than people expect. Balloon mollies have extra health baggage; prefer standard body types when possible.

If you only have a 10-gallon, choose guppies (or a betta), not a molly colony. Recipes: 10-gallon stocking · 20-gallon stocking.

Water chemistry — the real divider

Parameter	Molly priority	Guppy priority
Hardness	High — this is the hill to die on	Moderate is fine
pH	Alkaline (often 7.5+)	Flexible mid-range
Salt	Optional tool, not a cure-all	Rarely needed

Mollies from hard-water shops crash in RO/soft Amazon-style tanks. Fix mineral content before blaming disease. Guppies forgive more, though quality-bred stock still wants cleanliness and stable heat.

Temperament and behavior

Both are peaceful with similarly sized community fish. Males may chase females for breeding.

• Guppy males harass females if the ratio is wrong — aim for 2–3 females per male or keep all males
• Mollies are bolder and larger; they can outcompete tiny shy fish at feeding
• Neither belongs with bettas as a safe default — flowing tails invite nipping (betta tank mates)

Diet

Guppies: flakes/pellets + frozen treats.
Mollies: more herbivorous bias — spirulina, veggie flakes, blanched vegetables, algae wafers, plus occasional protein.

Hungry mollies pick at soft plants; well-fed mollies with algae and veg do better.

Breeding and population control

Both birth free-swimming fry.

• Guppies: fry every ~28–35 days under good conditions
• Mollies: slightly longer intervals, larger mothers, fewer but bigger broods sometimes

Plan for:

• All-male tanks, or
• Heavy planting / breeder box ethics, or
• Rehoming plan

Ignoring breeding turns a pretty tank into a bioload bomb. Stocking math: stocking guide.

Disease and hardiness notes

• Poor guppy store stock can arrive with parasites — quarantine helps
• Mollies show shimmies when cold, soft, or dirty — correct water before random meds
• Both need cycled tanks; livebearers do not survive new tank syndrome better than anything else — new tank syndrome

Can mollies and guppies live together?

Yes in a 20–30+ gallon with hard-enough water for the mollies, good filtration, and sex-ratio planning. The water must satisfy the molly, not only the guppy. Soft acidic water = bad molly tank even if guppies look fine.

Who should choose which?

Choose guppies if you:

• Have 10–20 gallons
• Want smaller, flashier fish
• Have softer or average tap water
• Are okay managing fry or running males-only

Choose mollies if you:

• Have 30 gallons (or a solid 20 for a small group)
• Already keep hard, alkaline water
• Want larger livebearers that graze
• Will feed vegetable matter on purpose

Choose neither if:

• The tank is uncycled
• You wanted a “no maintenance” bowl
• You planned to mix them with a male betta in a nano

Common mistakes

• Mollies in soft water “Amazon” communities
• Mixed-sex guppies with no fry plan
• One molly in a 5–10 gallon
• Assuming salt fixes every molly problem
• Skipping quarantine on cheap livebearers

Related resources

• Molly care
• Guppy care
• Platy vs molly
• Best beginner aquarium fish
• 20-gallon stocking
• Choosing tank mates

Short version: Guppies are the flexible small livebearer. Mollies are the hard-water, bigger-tank livebearer. Match the water and volume first — color second.