Can otocinclus live with firemouth cichlids?

Verdict
Yes

Yes. Otocinclus and firemouth cichlids get along in the same tank. Both are fine at 75–79°F. Plan on at least 30 gal.

OtocinclusOtocinclus72–79°F · pH 6–7.5 Firemouth cichlidFiremouth cichlid75–86°F · pH 6.5–8

Why it works

Water side by side

55°60°65°70°75°80°85°90°Both: 75–79°Otocinclus 72–79°Firemouth cichlid 75–86°
The green band is the temperature range both can live in.
456789Both: 6.5–7.5Otocinclus 6–7.5Firemouth cichlid 6.5–8
The green band is the pH both tolerate.

Size to scale

Otocinclus · 2 inFiremouth cichlid · 6 in0"1"2"3"4"5"6"
Adult length in an aquarium. A fish can swallow anything that fits in its mouth, roughly a third to half its own length for predators.

Care at a glance

OtocinclusFiremouth cichlid
Temperature72–79 °F75–86 °F
pH6–7.56.5–8
Adult size2.0 in6.0 in
Minimum tank20 gal30 gal
TemperamentPeacefulSemi-aggressive
Swims atBottomBottom
GroupGroups of 6+Alone or in pairs
DietHerbivoreOmnivore

Better tank mates for otocinclus

All otocinclus tank mates

Better tank mates for firemouth cichlids

All firemouth cichlid tank mates

Questions

Can otocinclus live with firemouth cichlids?

Yes. Otocinclus and firemouth cichlids get along in the same tank. Both are fine at 75–79°F. Plan on at least 30 gal.

What temperature suits both otocinclus and firemouth cichlids?

75–79 °F. Otocinclus need 72–79 °F, firemouth cichlids need 75–86 °F.

What size tank do otocinclus and firemouth cichlids need together?

At least 30 gal, set by the firemouth cichlid. Add space for each extra fish.