An aquarium is the nearer relative, not the medicine cabinet: both hold something suspended and still, both discourage touching, and both use a color that never quite occurs in nature to signal that the water inside is not for drinking. Barbicide, the brand that built the entire category, dyes its solution the specific blue of a swimming pool photographed in a brochure, chosen so a barber can tell at a glance whether the jar needs changing. Two combs stand submerged near the base, tines down, doing nothing for the ten or twelve minutes between customers that make up their entire working rest.
The liquid is a disinfectant solution, a quaternary ammonium compound diluted to something close to hospital grade concentration, meant to be replaced on a schedule most shops treat as a suggestion. Jars have been seen going a month without a change, the blue gone faintly grey at the bottom where hair and skin cells settle, and just as often changed daily in shops that otherwise cut corners everywhere else, an inversion that has more to do with which owner happens to notice than with any cleaning standard. The screw lid rarely gets used during business hours; the jar stays open on the counter until closing, then seals overnight against evaporation and reopens the next morning at the same faint tide line. It remains, by any measure, the least glamorous object on the counter and the one doing the most consistent work.