How long have you had the otos? What size tank and how established is the tank?
They are very sensitive. For 6, I would presume you to have at least a 75 gallon tank. They require as much natural algae as possible and many will not accept processed food as a supplement. They die of starvation.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~