Best Molly Tank Mates: Compatible Species

Understanding what mollies actually need

Before you pick tank mates, get the molly's baseline right. Poecilia sphenops, P. latipinna, and P. velifera - the three species you'll find in most shops - are livebearer fish native to Central American coastal waterways, which means brackish influence is baked into their genetics. They don't need salt in a community tank, but they do need hard, alkaline water. Target pH 7.5-8.2, general hardness 15-25 dH, and a temperature of 24-28 °C (75-82 °F).

That water chemistry requirement is what rules out the majority of popular community fish. Most of the "compatible" lists you'll find online were written without anyone actually checking whether the recommended species can handle the same mineral content mollies need. Soft-water fish placed in hard, alkaline setups show chronic stress, faded colour, and shortened lifespans - even if they technically survive for a while.

Get your parameters stable before adding any community fish. If you're not sure your cycle is solid, read the guide on how to cycle a fish tank first - introducing tank mates to an uncycled or unstable tank creates problems that water chemistry alone can't fix.

Tank size and group dynamics

Eighty litres is the floor, not the goal. A group of four to six mollies with a supporting cast of Corydoras and mid-water schoolers will be genuinely comfortable in 120-150 L. More space reduces the harassment that male mollies direct at females, gives schooling fish room to behave naturally, and keeps the bioload manageable.

On group ratios: keep two or three females for every male. A single female with two males will be harassed to exhaustion and death. This isn't an edge case - it's a predictable outcome that gets repeated in tanks everywhere. Males are relentless. Spreading the attention across multiple females is the only practical solution short of keeping a single-sex group.

The best molly tank mates

Other livebearers: platies, swordtails, guppies

The easiest wins are other members of the livebearer family. Xiphophorus maculatus (platy) and Xiphophorus hellerii (swordtail) share virtually identical water requirements - pH 7.0-8.0, hard water preferred - and have comparable temperaments. They're active mid-water fish that won't compete directly with mollies for territory and won't be intimidated by them either.

Guppies (Poecilia reticulata) work well in principle because they're close relatives with overlapping water needs. The practical issue is male guppies. Their flowing fins can attract nipping from more boisterous mollies, and in a heavily stocked tank that harassment compounds. In a spacious, well-planted tank with plenty of break-up cover, guppies and mollies coexist reliably. In a bare 80 L tank, you'll sometimes see problems.

One thing to be aware of with all livebearer mixes: they breed prolifically. If you're mixing species, you won't get hybrid fry (these species don't typically hybridise in standard setups), but you will get a tank that fills up fast. Have a plan for fry before the fish breed - because they will.

Corydoras catfish

Corydoras are one of the cleanest choices for a molly community tank. They occupy the bottom of the tank, they're peaceful, and many species do well at pH 7.0-8.0 in moderate hardness. Corydoras paleatus (peppered cory) and C. aeneus (bronze cory) are both robust enough to handle the harder end of the water range that mollies prefer.

Keep Corydoras in groups of six or more - they're shoaling fish and show stress when kept in smaller numbers. A group of six corys on the bottom of a 120 L tank with mollies and some mid-water schoolers is a visually active, well-layered community that works.

Danios and rasboras

Several danio and rasbora species handle harder water well enough to work alongside mollies. Danio rerio (zebra danio) is highly adaptable and will tolerate a wide pH and hardness range without complaint. Devario aequipinnatus (giant danio) is another solid choice for larger tanks - active, robust, and unbothered by molly social behaviour.

Rasboras are more variable. Trigonostigma heteromorpha (harlequin rasbora) prefers slightly soft, acidic water and sits at the edge of what mollies are comfortable with. It can work if your water lands in the neutral zone around pH 7.0-7.2, but it's not the first pick. Rasbora trilineata (scissortail rasbora) is more flexible and handles moderate hardness better.

Whatever danio or rasbora you choose, keep them in groups of at least eight. Their schooling behaviour is the point - a group of three looks nervous and stressed, not decorative.

Peaceful plecos and other algae eaters

Bristlenose plecos (Ancistrus sp.) are a practical addition. They stay small (10-15 cm), handle hard alkaline water fine, and clean algae off glass and decor. Avoid common plecos (Pterygoplichthys pardalis) - they reach 40-50 cm and will outgrow almost any home tank.

Nerite snails also work well in molly tanks. They're effective algae grazers, they prefer hard water (it prevents shell erosion), and they won't be bothered by mollies. Mystery snails are another option, though mollies occasionally nip at their antennae in smaller tanks.

Fish to avoid and why

Bettas

The betta-molly combination gets recommended more than it should. The problems are real and consistent: male bettas are territorial and will treat the active, nippy nature of mollies as a provocation. Mollies will nip at betta fins. The betta will respond. The result is chronic stress for both fish, and usually injury to at least one of them. Some aquarists report it working in large, heavily planted tanks - but it's a marginal case, not the norm. Build your tank around a reliable community rather than hoping this particular combination lands on the right side of the odds.

Blackwater and soft-water specialists

Discus, most dwarf cichlids from the Apistogramma genus, cardinal tetras, and altum angelfish all want soft, acidic water - typically pH 5.5-6.8, hardness under 8 dH. That's the opposite end of the spectrum from what mollies need. You cannot find a compromise water chemistry that satisfies both sides. These species shouldn't share a tank with mollies.

Large or aggressive cichlids

Most New World cichlids - Oscars, Jack Dempseys, green terrors - will simply eat or bully mollies. Mollies are not fighters. Even species that are technically "medium" aggression can make life difficult. Stick to peaceful or semi-peaceful community fish unless you're building a specific cichlid setup.

What most guides miss: salt tolerance and the "brackish option"

Mollies are unusual among freshwater fish in that they tolerate - and in some cases actively benefit from - small amounts of aquarium salt (1-2 tablespoons per 10 L). Some experienced keepers run molly-specific tanks as lightly brackish setups to reduce disease susceptibility and improve fin quality. If you go this route, your tank mate options narrow significantly. Most freshwater community fish don't tolerate salt well at all. Corydoras in particular are sensitive to it and shouldn't be kept in salted water.

For a true community tank, keep it fully freshwater. You'll have more compatible species to choose from and won't stress animals that can't handle the salt. The brackish option is worth knowing about, but it's for a molly-specific setup - not a community tank.

Putting it all together: a practical stocking example

For a 120 L tank at pH 7.5-8.0, 18-22 dH, 26 °C (79 °F):

• 6 mollies (2 males, 4 females) - active mid-water layer
• 6 platies - same layer, different body shape and colour
• 8 zebra danios - upper water, fast and dynamic
• 6 bronze Corydoras - bottom layer
• 2 bristlenose plecos - algae control on hard surfaces

That's a stocked but manageable community. Monitor ammonia and nitrite weekly until the tank is stable, and do a 25-30% water change weekly. For detailed guidance on aquarium water parameters and understanding what your test results mean, that guide covers the full picture. For how often to change water as your bioload increases, see how often to do water changes.

Before you finalise your stocking list, run it through the free stocking calculator to check your tank isn't overstocked. Bioload isn't just about fish count - it's about the waste output per species, and mollies produce more than their size suggests.