Mollies with shrimp
Mollies are livebearers in the genus Poecilia. The two species behind most pet-store mollies are the short-finned molly, P. sphenops, native from Venezuela and Colombia north to Mexico, and the sailfin molly, P. latipinna, from the Carolinas to Veracruz; most store fish are hybrids of the two in colors such as black, dalmatian, gold dust and lyretail (Seriously Fish, Aquarium Co-Op). Aquarium Co-Op notes their flat mouths are good at tearing off hair and string algae, which is why mollies get suggested for planted tanks and, from there, for shrimp tanks.
Mollies at a glance
| Mollies (Poecilia sphenops, P. latipinna and hybrids) | |
|---|---|
| Adult size | 80 mm (3.1 in) SL short-finned; 125 mm (5 in) SL sailfin (Seriously Fish); 4–5 in (Aquarium Co-Op) |
| Temperature | 70–82 °F (21–28 °C) short-finned; 70–79 °F (21–26 °C) sailfin (Seriously Fish) |
| pH | 7.0–8.5 (Seriously Fish) |
| Hardness | 15–30 dGH short-finned; 15–35 dGH sailfin (Seriously Fish); hard, alkaline water (Aquarium Co-Op) |
| Tank | Base of 36 × 12 in (90 × 30 cm), about 21 US gallons, for short-finned; 30 × 15 in, about 23 US gallons, for sailfin (Seriously Fish) |
| Group | Several females per male, because males chase females persistently (Seriously Fish) |
| Diet | Omnivore that eats zoobenthos and detritus in the wild, with plenty of vegetable matter (short-finned); wild sailfins are almost entirely herbivorous (Seriously Fish) |
| Temperament | Peaceful short-finned; sailfins may pick on smaller fish as they mature, and males can be aggressive to each other (Seriously Fish) |
Will mollies eat shrimp?
- Shrimplets: yes. Seriously Fish says adult short-finned mollies prey on their own young, and a newborn shrimp is far smaller than a molly fry. In our reading, any shrimplet out in the open is food.
- Adult shrimp: often ignored, but not reliably. A 4 to 5-inch molly has a mouth big enough to try, and Seriously Fish says sailfins may pick on smaller tankmates as they mature (our reading for shrimp).
- Molting shrimp: soft and slow, so they're the adults most likely to be picked at (our reading).
- Food competition: Practical Fishkeeping says fish are quicker to food than shrimp and that algae-grazing fish compete for the biofilm young shrimp depend on. Mollies are keen algae grazers (Aquarium Co-Op).
Practical Fishkeeping says larger livebearers are among the fish where it's "not a matter of if but when" they eat your shrimp. Mollies sit at the large end of the common livebearers, between platies and swordtails (Aquarium Co-Op). Compare platies with shrimp and guppies with shrimp, or see the full cherry shrimp tank mates list.
Matching the water: a wide hardness gap
Seriously Fish says mollies must be kept in moderately hard or harder water with a basic pH, and that in soft or acidic water they weaken quickly, often shown by shimmying, fungus or clamped fins. It lists 70–82 °F, pH 7.0–8.5 and 15–30 dGH for short-finned mollies. The Shrimp Farm lists 65–80 °F, pH 6.8–8.0, GH 4–8 and KH 2–5 for cherry-type shrimp. By our comparison, temperature (70–80 °F) and pH (7.0–8.0) overlap, but there is a gap of about 7 dGH between the top of the shrimp range and the bottom of the molly range, wider than the gap with platies. There is no setting that suits both well, and a compromise risks unhealthy fish or poor shrimp molts (our reading). See shrimp GH and KH explained.
Do mollies need salt?
No. Seriously Fish says salt is not necessary for either species; the hard minerals, calcium and magnesium, are what matter. It also says short-finned mollies often do better in slightly brackish water and can even live in full marine conditions, and the sailfin is often called a brackish fish but lives perfectly well in hard fresh water. Cherry shrimp are freshwater animals, so don't add salt to a shared tank.
Tank size and the fry problem
- Size: at least a 30 to 36-inch tank of about 21 to 23 gallons (Seriously Fish), which is larger than most shrimp tanks.
- Fry: Seriously Fish says short-finned molly broods of up to 120 fry are not uncommon after about 8 weeks' gestation, and sailfins produce up to 50. More fish mean more waste and more mouths searching the moss (our reading).
- Weak strains: Seriously Fish says many store mollies are genetically weak and prone to disease after years of inbreeding, especially black mollies, and a sick or dead fish can foul a small tank (Practical Fishkeeping).
- Filter intake: cover it with a sponge to protect fry and shrimplets; see shrimp-safe filters.
Better options
- Breeding shrimp? Keep them in their own tank and move surplus adults elsewhere. This is our recommendation.
- Want a livebearer? Smaller ones are lower risk: see platies with shrimp, guppies with shrimp and Endler's livebearers with shrimp.
- Want algae control? Shrimp graze algae themselves; see algae in a shrimp tank.
- Already have mollies? Adult Amano shrimp are larger than cherry shrimp; see Amano shrimp care.
If you try it anyway
- Shrimp first. Practical Fishkeeping says to give the colony time to build up before adding fish.
- Dense cover at the bottom. See baby shrimp care and java moss for shrimp.
- Feed the fish well, since Practical Fishkeeping says well-fed fish reduce losses; include vegetable matter, as Seriously Fish advises.
- Acclimate slowly, since hard molly water will differ from most shrimp tanks. See how to drip acclimate.
- Quarantine the fish separately. Practical Fishkeeping says many fish medications are not safe for invertebrates. If fish get sick, ask a fish veterinarian or an experienced store about treatment that won't harm the shrimp. See how to quarantine and is copper toxic to shrimp.
Common mistakes
- Keeping mollies in soft water to suit the shrimp; Seriously Fish says they weaken quickly in it.
- Adding salt for the mollies in a shrimp tank.
- Buying a mixed group with no plan for the fry.
- Mixing short-finned and sailfin mollies, which Seriously Fish says interbreed readily.
Track it
Log temperature, pH, GH and TDS on the free Water Parameter Log printable so you can keep the tank in the range both species share.
Frequently asked questions
Will mollies eat cherry shrimp?
They will eat shrimplets, and a large molly may go after small or molting adults. Practical Fishkeeping says larger livebearers do not belong with shrimp. We rate mollies a high risk.
Can cherry shrimp breed in a tank with mollies?
A few babies may survive in very dense moss, but expect most to be eaten. Keep shrimp in their own tank if you want the colony to grow.
What water do mollies and cherry shrimp share?
About 70 to 80 °F and pH 7.0 to 8.0. Hardness does not overlap: Seriously Fish lists GH 15 to 30 for short-finned mollies and The Shrimp Farm GH 4 to 8 for cherry shrimp.
Do mollies need salt in a shrimp tank?
No. Seriously Fish says salt is not necessary; hard minerals are what mollies need. Do not add salt to a cherry shrimp tank.
Are mollies or platies better with shrimp?
Platies. They are smaller and need less extreme hardness. We rate platies a moderate risk and mollies a high risk.
Can mollies live with Amano shrimp?
Adult Amanos are larger than cherry shrimp, so in our reading they are safer with mollies, though Amanos do not breed in fresh water.
Sources: Seriously Fish, Poecilia sphenops (short-finned molly) (Venezuela and Colombia to Mexico; 80 mm SL; base 90 × 30 cm, about 21 US gal; 21–28 °C, pH 7.0–8.5, 15–30 dGH; must have moderately hard or harder water; salt not necessary; does better slightly brackish and tolerates marine; peaceful; omnivore needing vegetable matter; broods up to 120; adults prey on young; weak inbred strains); Seriously Fish, Poecilia latipinna (sailfin molly) (Carolina to Veracruz; 125 mm SL; base 76 × 38 cm, about 23 US gal; 21–26 °C, pH 7.0–8.5, 15–35 dH; wild fish almost exclusively herbivorous; may pick on smaller fish; up to 50 young; often called brackish but fine in hard fresh water); Aquarium Co-Op, Top 10 livebearer fish to try breeding (mollies 4–5 in, bigger than platies, smaller than swordtails; mostly hybrids; need hard, alkaline water; flat mouths for tearing off hair and string algae); Practical Fishkeeping, What can I keep with shrimp? (larger livebearers don't belong with shrimp; fish win at feeding time and compete for biofilm; dead fish and medications endanger shrimp); The Shrimp Farm, Red rili shrimp tank set-up and care (cherry-type range 65–80 °F, pH 6.8–8.0, GH 4–8, KH 2–5).