Do Otocinclus or Nerite Snails Actually Clean Up Brown Algae?

Written 2026-11-03

Both nerite snails and otocinclus catfish eat brown algae (diatoms) and bring it down. But timing matters more than appetite. Running the Water Kiwoogi simulator on a 60cm standard tank with plain black sand and no livestock at all, the diatom index (0–100) climbed to 64 by day 60 and was still rising. Add two nerite snails on day one — before the tank is even cycled — and the index stayed at 0–1 for all 60 days. Add four otocinclus the same way, on day one, and they never got the chance to eat much of anything: their own ammonia and nitrite wiped out all four by day 18.

Why diatoms don’t clear on their own

Diatoms feed on dissolved silica, which both fresh substrate and tap water release (tap water alone carries about 8mg/L). Unlike green algae, they grow fine even under weak light. Left alone they only decay about 4% a day, and what really pushes them out is a mature biofilm on the glass and substrate — and biofilm grows by eating organic matter from fish waste and leftover food. A tank with no fish and no food barely grows any biofilm, so as water changes keep resupplying silica from tap water, diatoms just keep climbing. In the plain black-sand, no-livestock tank, diatoms were visible by day 6 and still climbing past day 60.

Nerite snails can go in early

Nerite snails carry a very light bioload (0.5 per snail, about half a neon tetra). That means adding them before a tank is cycled doesn’t push ammonia or nitrite very high. Two nerite snails added on day one to the tank above kept ammonia under 0.22ppm and nitrite under 1.07ppm (test-kit scale) for the full 60 days, and both snails survived. Over that time the diatom index never topped 1 — it got eaten before it had a chance to become visible.

Otocinclus is risky that early

Otocinclus has the highest algae-eating rate of any species in this simulator (3.2 per fish), but it has low ammonia and nitrite tolerance and needs to be kept in groups of four or more. Add four otocinclus on day one to the same tank, and deaths start on day 16, with all four gone by day 18. The cause wasn’t algae — it was the ammonia and nitrite (nitrite peaked at 3.4ppm) the fish themselves produced, more than an unestablished filter colony could handle. The catalog entry for otocinclus already says this: “dies first if added to an uncycled tank.”

Safe once another fish has settled in

The same four otocinclus tell a completely different story when added to a tank where six neon tetras, helped by a bacteria starter, have already been established for 20 days. All ten fish — six neon tetras and four otocinclus — survived the full 60 days, and the diatom index, which had been hovering around 10, cleared to 0 by day 25. That’s a bit faster than the same tank clears on its own in the 30s without otocinclus. The takeaway: otocinclus is safe once a filter colony is already established, whether by other fish or by a bacteria starter.

Both species also share a practical plus: neither breeds explosively in freshwater, so their numbers stay where you put them. If you’re setting up a new tank, nerite snails can go in first; save otocinclus for after another fish or a bacteria starter has gotten the filter colony going. If you’re unsure, check with a local fish store or an experienced keeper.

The button below is the 60cm standard tank · 2 nerite snails added on day 1 tank from this guide.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.