What clean-up crew should I add first, and why?
When: Right after the pre-livestock water change, once the cycle is confirmed complete and stable
Short answer: Snails, hermit crabs, and other grazing invertebrates go in first because they're hardy, add almost no bioload, and eat the diatom/algae blooms of the "ugly phase" that hits right as your tank matures. Add a small starting group, then let algae growth tell you if you need more — don't dump in a large crew all at once.
The details
Why clean-up crew (CUC) first. A snail or hermit crab produces far less waste than a fish of similar size, so it's a low-risk way to add the first life to a freshly-stabilized tank. Their job is functional, too: they graze the diatom and algae blooms that are a normal part of a new tank's "ugly phase," keeping glass, rock, and sand cleaner while your tank finishes maturing.
What to get. Snails (trochus snails are a hardy, classic choice — they're able to right themselves if a current or a fish tips them over) plus hermit crabs are the core of most crews. Other crabs add specific jobs: emerald crabs are known for eating bubble/hair algae, porcelain crabs are filter feeders, and arrow crabs eat bristleworms but can be semi-aggressive toward small tankmates — read up on a species' role and compatibility before adding it.
No fixed head-count formula. Our sources don't give a single snails-per-gallon ratio. Start with a modest group, watch how quickly algae reappears, and add more gradually rather than buying a large crew sized for a "finished" tank that isn't there yet.
They still need proper acclimation. Invertebrates are especially sensitive to sudden salinity changes — more so than fish — so don't skip the drip-acclimation step covered in the next card.
The numbers
| Step | Target |
|---|---|
| When to add | After the post-cycle water change, once parameters have held a week |
| Bioload | Near zero |
| Starting group size | No fixed ratio in our sources — start small, add gradually |
| Acclimation | Drip acclimate (see next card) — inverts are salinity-sensitive |
Common mistakes
- Skipping CUC and going straight to fish. Nothing is left to eat the ugly-phase algae bloom.
- Float-and-dump instead of drip acclimating. Inverts handle salinity swings worse than fish.
- Buying a huge crew on day one. An oversized crew sized for a mature tank's algae load will start starving once the ugly phase clears.
When to worry
- Normal: crabs and snails hiding for the first day or two, and diatoms/algae still visible while the crew works on it.
- Worry: any snail or crab dying within days of introduction — check it was drip acclimated properly and that ammonia/nitrite are still reading 0.
What's next
Once your clean-up crew is settled, move on to picking your first fish: how-to-choose-a-healthy-fish-at-the-store.
- Adding a full fish stock before any clean-up crew — nothing is eating the diatom/algae bloom
- Dumping in dozens of snails/hermits at once instead of a small starting group
- Reef Tank Cycling Research (internal)
- Coral Reef Knowledge Base (internal) — Invertebrate Care / Picking the Right Crabs