Reef Tank Inverts: How Many & How Often

Most people buy a clean-up crew sized for a tank that doesn’t exist yet. You set up a new system, the diatoms show up, you panic, and you order the pack that says “up to 75 gallons.” Thirty-five animals arrive. Two months later half are dead and you assume they were weak stock. They weren’t. You starved them. A new tank doesn’t have enough food to support thirty-five grazers, and a snail that runs out of algae doesn’t move out — it dies in a rock crevice and rots there, which makes the nutrient problem slightly worse. This is the article about numbers. For the species-by-species breakdown, read Clean-Up Crew 101 → first. This one covers how many to start with and how often you’ll actually be buying more.

First — what a clean-up crew is not

It is not an algae treatment. If hair algae is covering your rockwork, you have a nutrient problem, a lighting problem, or a feeding problem. Snails will not fix it. Add a hundred and you’ll have a hundred snails and the same hair algae, because it’s growing faster than they graze and the cause hasn’t moved. A clean-up crew keeps a clean tank clean. It’s groundskeeping, not an emergency service. Fix the cause first — start with our algae ID guide → — then stock the crew to hold the line.

The animals that earn their place

Snails

Trochus are the best all-around snail in the hobby and it isn’t close. They graze film and short hair algae off glass and rock, they’re completely reef safe, and they can right themselves when they fall. They’ll breed in a healthy tank, so the crew partly restocks itself. Cerith are the multitaskers — glass, rock, and sand, and they burrow, so they work the substrate at night. Small, cheap, and they reproduce in-tank. Nassarius are not algae eaters. They live buried and erupt out of the sand the second food hits the water. Their job is leftover meaty food and detritus. Every tank with a sand bed should have some. Astrea graze well but can’t flip themselves back over. One on its back is on a clock. Fine snail, higher babysitting cost. Turbo are heavy equipment — one clears more algae than five trochus, and will also bulldoze unglued frags off your rock. Once they’ve eaten everything they starve, because a tank clean enough to look good doesn’t feed a turbo. Use them to knock down a bloom in a big system, not as permanent stock in a nano.

Hermits — optional, and that’s not a typo

Blue-leg and scarlet hermits scavenge leftover food out of gaps snails don’t reach. Scarlets are the more peaceful of the two. The problem is shells. Hermits grow, need bigger shells, and a hermit that wants one will kill a snail to get it. If you keep hermits, throw a dozen empty shells of assorted sizes in and keep doing it. Plenty of experienced reefers skip hermits entirely and don’t miss them.

Emerald crab — for bubble algae specifically

Mithraculus sculptus, a Caribbean animal and one we collect locally. It’s one of the very few things that actually eats Valonia. The caveat is real. An emerald crab that runs out of algae becomes an opportunist and will pick at zoas and occasionally LPS. One per tank, keep it fed, pull it if it misbehaves.

Shrimp

Skunk cleaner shrimp are the best all-round reef shrimp — aggressive scavengers that set up cleaning stations and service your fish, bold enough to take food from your fingers. Peppermint shrimp (Lysmata wurdemanni, an Atlantic species) are the Aiptasia answer, emphasis on answer rather than guarantee. Some hunt Aiptasia relentlessly; some ignore it and eat your zoas instead. The species also gets substituted constantly at the wholesale level, which is half of why results vary. Buy from someone who knows what they actually shipped you. More on pest anemones in Common Reef Tank Pests → Coral banded shrimp are beautiful and will kill other shrimp. Not a community animal.

Sand bed crew

Fighting conch — a Florida native and genuinely excellent. It plows the top layer of sand continuously, leaving the bed cleaner and better oxygenated than any snail manages. Needs a real sand footprint; don’t put one in a 20 gallon. Copepods and amphipods are the layer under everything else, processing detritus at a scale you can’t see and feeding everything from your corals to your wrasses. Seed once the tank has cycled and dose phyto. See Copepods 101 →

Sold as clean-up crew. Skip them.

• Sand sifting starfish — Astropecten strips your sand bed of every living thing in it, then slowly starves. In anything under a few hundred gallons it’s a net negative.
• Green brittle star — Ophiarachna incrassata catches and eats fish at night. Other serpent and brittle stars are fine.
• Urchins in small tanks — outstanding turf grazers and absolute bulldozers. They’ll carry your frags around stuck to their spines.
• Sea cucumbers — effective, but some species release toxins when they die or get shredded by a powerhead. Wrong risk for a first tank.

How many to start with

Forget “one snail per gallon.” That rule is decades old and it’s the single biggest cause of clean-up crew die-off. Start at roughly one snail per four gallons, mixed across two or three species, and scale from what you actually see.

Tank size	Starting snails	Hermits (optional)	Sand crew	Shrimp / crab
20 gal	5–6 mixed	2–3	3 nassarius	1 cleaner shrimp, later
40 gal	10–12 mixed	3–5	5 nassarius	1 cleaner shrimp
75 gal	18–20 mixed	5–8	8 nassarius, 1 conch	1–2 shrimp, 1 emerald
120 gal+	30–35 mixed	8–12	12 nassarius, 1–2 conch	2–3 shrimp, 1–2 emerald

Read those as ceilings for a mature tank, and start at half in a new one. Order of operations. Pods first, once the tank has cycled and biofilm has formed. Snails when the diatom bloom shows up, usually a few weeks in. Hermits and shrimp after that. Conch and emerald last, once there’s enough in the tank to feed them. Then watch the glass. Scraping film algae every other day means add five more snails. Snails sitting on bare rock with nothing to graze means you’re already at your ceiling.

When do you replace them?

You don’t replace a clean-up crew on a schedule. You top it up when the tank tells you to. Reasonable lifespans in a stable system:

• Snails — 1–3 years, and trochus and cerith often breed in-tank fast enough to partly replace themselves
• Hermits — 2–4 years
• Cleaner shrimp — 2–3 years, sometimes longer
• Emerald crab — 2–4 years
• Fighting conch — several years with enough sand to work
• Pods — self-sustaining unless a hungry wrasse or dragonet wipes them out

Most tanks need a modest top-up every six to twelve months. Not a whole new crew — a handful of snails to backfill.

Troubleshooting

Losing snails every couple of months. Don’t buy more snails. Find what’s killing them, in this order:

1. Acclimation. Inverts have effectively zero tolerance for salinity swings — far less than fish. Drip acclimate over at least an hour. Most “weak stock” is a bag that got dumped in over five minutes.
2. Starvation. The most common cause and the least diagnosed, because it looks like nothing — the tank just quietly has fewer snails every month.
3. Alkalinity swings. Inverts feel a moving alkalinity before your corals visibly do.
4. Copper. If that tank was ever treated with a copper-based medication, it’s in the rock and sand and will keep killing inverts for years. Copper and inverts never coexist.
5. Hermits taking shells. Losing snails and finding empty intact shells isn’t a mystery. Add spare shells.

Empty shells with the animal still inside, all at once. Check salinity and temperature first — a heater failure or an ATO dump kills the crew before it kills the fish.

Parameters worth keeping steady

Stability beats perfection. Inverts punish swings harder than corals do.

• Salinity: 1.025–1.026
• Temperature: 76–78°F
• Alkalinity: 8–9 dKH
• Calcium: 400–450 ppm
• Magnesium: 1300–1400 ppm
• Copper: zero, permanently

Where ours come from

The emerald crab, the peppermint shrimp, the fighting conch and several of the snails here are western Atlantic and Caribbean animals — the same water we work in. We collect them from Keys waters under our Florida commercial saltwater products license, SP-210504 (MLD-51), one of a limited number of grandfathered marine life endorsements still in existence. Inverts that haven’t spent a week crossing the Pacific in a bag arrive in noticeably better shape.

Talk to us before you buy

We’re a working coral farm and saltwater shop in Key West — not a warehouse. If you’re not sure whether a piece suits your tank, ask first. We would rather talk you out of the wrong coral than replace it for you later.

• Book a live consultation → — 30 or 60 minutes with us, scheduled right here on the site. Bring photos of your tank.
• Visit the shop → — Mon & Fri 10:30–6:30, Sat 11–6, Sun 11–5. Tue–Thu closed to the public for fish collecting and aquarium maintenance. Come see the systems and pick your own piece.
• Send us a message → — if you would rather write it out.

The crew a tank needs depends on what is actually growing in it. The Reef Crew Prescription matches snails, hermits and shrimp to the problem you can see — hair algae, film on the glass, food rotting in the sand bed — instead of a fixed package that ignores your tank.