Abstract The regulation of a connected tank system which is used for mixing dyes, in aqueous solution, is considered. Novel modelling, in matrix impedance (P,Q,R) form, from which the state space and transfer function, admittance, realisations of the system are generated, is proposed. Independent level control of the mixing tanks is investigated. Steady-state non-interaction of the closed-loop system is required, using pneumatically operated valves, each of which has finite time delays. Optimum operating conditions are specified. Minimum closed-loop control effort and almost critically damped response transients are achieved.