Peaceful to a fault
There is no aggressive bone in an otocinclus. They do not chase, nip, posture, or defend territory; their entire social life consists of grazing near their own kind and drifting between leaves and glass. In a community aquarium they are the fish least likely to cause a problem, and their small adult size — barely an inch and a half — means they never crowd or threaten even the smallest tankmates. If the question is whether otos will disturb your community, the answer is simply no.
The risk runs the other way
A peaceful fish still fails in the wrong community, and otos fail in two predictable ways. The first is competition: they are slow, methodical grazers, and fast mid-water feeders or pushy bottom-dwellers will strip every wafer and vegetable slice before an oto arrives. The second is predation and harassment — angelfish, larger cichlids, aggressive barbs, and big loaches are all capable of treating an oto as either a snack or a toy. Even fin-nipping species that never touch the otos directly can stress them into permanent hiding, and a hidden oto is usually an underfed one.
Tankmates that bring out their best
Build the community around small, calm species and the otos become visible, active fish rather than shadows. Ember and neon tetras, harlequin rasboras, small corydoras, honey gouramis, and cherry shrimp all share the oto's unhurried pace, and none compete unfairly for grazing food. Their own kind matter most of all: six or more otos form a loose shoal that moves around the tank together, and that confidence transforms their behavior. Stock the group first or all at once rather than adding single fish over months, so no individual spends its early weeks alone.
Feeding peace at the bottom of the tank
Keeping the peace in an oto community is largely a feeding strategy. Scatter sinking algae wafers in more than one spot after the main feed, drop blanched zucchini near their favorite leaves, and let some soft algae persist on the back glass and hardscape instead of scrubbing every surface clean. Watch bellies rather than behavior at feeding time — rounded undersides across the group mean the community balance is working, while thinning fish in a tank full of lively tankmates means the otos are losing the quiet competition nobody sees.
Reading the mood of the tank
A settled oto community has a recognizable rhythm: grazing in the open during the day, resting together on broad leaves, occasional dashes to the surface. Fish that have vanished into the plants for days, or that only emerge frantically at night, are living in a community that frightens them, and the fix is changing the mix of tankmates rather than the otos themselves. Before your next purchase, a good local aquatics shop found through KinsLe's Pet Services Directory will happily talk through how a new species is likely to behave alongside your grazing school.