Molly Tank Setup: A Real 20 Gallons, Sturdy Filtration, and the Salt Question

Mollies need more room and sturdier filtration than a guppy from day one, and they raise a question guppies never do: whether the water needs a little aquarium salt in it.

Tank Size

20 gallons is the real minimum for a small group, a step up from the 10-gallon tank that works fine for guppies. Mollies simply run bigger as adults, and a colony that starts breeding adds to the bioload fast. Sailfin varieties, or any growing group, do better in 30 gallons or more. Go long and rectangular rather than tall, and use a lid, mollies are capable jumpers.

A 20-gallon long tank is a solid starting point for most first-time molly keepers.

Temperature

A tropical range of 72 to 82°F works well. Stability matters more than hitting an exact number, avoid swings larger than about 2°F in a 24-hour period, since temperature instability is one of the more commonly cited triggers behind the molly "shimmy." A heater is necessary unless your room stays consistently warm year-round.

Water Chemistry

Mollies do best with a pH of 7.5 to 8.5 and medium-hard to hard water. If your tap water runs soft, a mineral supplement, crushed coral or a commercial buffering product, can bring it into range, and the freshwater pH, GH and KH guide explains why KH is the parameter doing the work there and why dosing pH-down against a hard buffer crashes tanks.

The Salt Question, Answered Honestly

This is the detail that separates molly care from guppy care, and it deserves a straight answer rather than a repeated hobbyist talking point. Mollies are a brackish-tolerant, euryhaline species: peer-reviewed research on sailfin mollies shows they can tightly regulate their internal salt balance up to roughly 65 parts per thousand, well beyond typical seawater strength, without major physiological stress. That's real biology, not an old wives' tale.

Where it gets genuinely contested is whether a healthy freshwater display tank needs salt added as standard practice. Some sources recommend a small maintenance dose. Others, including longtime livebearer specialists, argue mollies "adapt to" salinity rather than requiring it, and that sufficiently large, frequent water changes and good live plant cover accomplish the same thing salt is usually reached for, without salt's real downsides: it can inhibit live plant growth and harm snails at higher concentrations. Salt does show up specifically in shimmy treatment, a commonly cited dose there is 1 tablespoon of aquarium salt per 5 gallons in a hospital tank, but that's a targeted treatment context, not necessarily a standing requirement for every molly tank.

If you do decide to add salt, the type matters. Use plain aquarium salt, additive-free sodium chloride, never a marine or reef salt mix, which carries buffers formulated for full saltwater tanks, and never table salt, which contains iodine and anti-caking agents that aren't meant for fish.

Filtration

Mollies need more real filtration capacity than guppies. They're bigger fish producing more waste, and a breeding colony compounds that quickly. For a standard or growing group, a hang-on-back filter or a stronger canister filter handles the bioload comfortably. PetMD's general guidance is a filter that can turn over the full tank volume at least four times an hour.

If fry survival is the priority instead, for example a dedicated breeding or nursery tank, a gentle sponge filter is the better call, since it won't pull small fry into strong current the way a power filter can. Which one you pick is really a question of what you're optimizing for, raw filtration capacity for a larger display group, or fry-safe gentleness for a breeding setup.

Substrate

Fine gravel or sand both work well. Avoid sharp or coarse gravel, which can damage fins, especially on longer-finned sailfin and lyretail varieties.

Lighting

A basic LED light on an 8 to 10 hour daily cycle is sufficient. It isn't strictly necessary for the fish's health, but it supports live plants and makes for better viewing.

The Two Non-Obvious Requirements

Cycle the tank before adding fish. Building the beneficial bacteria colony that processes waste takes 2 to 4 weeks with a fishless cycle, or somewhat less with a bottled bacteria product. Skipping this step is one of the most common beginner mistakes in the hobby generally, and it's a fast route to the ammonia-driven stress that shows up as shimmy.

Always use a water conditioner on tap water before it goes in the tank, dechlorinating and neutralizing chloramine and heavy metals that are otherwise harmful to fish, and chlorine exposure specifically is one of the recognized shimmy triggers. A water test kit lets you confirm the cycle actually finished and catch drift afterward.

Dense live plants aren't just decoration in a molly tank either, they're a real population-control lever. Molly parents don't look after their fry, and adults will eat them given the chance. If you're allowing your mollies to breed, dense live plants give fry somewhere to hide from adults, meaningfully improving survival. If you'd rather your population stay naturally controlled, skipping dense cover lets normal predation keep numbers in check instead, the same lever our Guppy tank setup guide covers for that closely related species.

The Molly encyclopedia profile has more on the species itself.

Sources & Further Reading