Using the double-plasma resonance model of solar radio zebras, we analyze five models of the magnetic field and density in the zebra source region. We present analytical relations of zebra-stripe frequencies depending on the gyro-harmonic number. By fitting of observed zebra-stripe frequencies using model frequencies, we find that the determined gyro-harmonic number and corresponding magnetic field depend on the model used. We show that all previously analyzed zebras, where the absolute value of the difference between neighboring zebra-stripe frequencies increases with respect to increasing frequency, can be well fitted by the model with exponential dependencies of the magnetic field and density or by the model with smaller gradients of both of these variables. Although these models give different results, their more sophisticated versions give more similar results. We also present the models that can fit the zebras, if observed, where the absolute value of the difference between neighboring zebra-stripe frequencies decreases with respect to increasing frequency. We check all these models by a fitting of the zebra-stripe frequencies observed in the 21 June 2011 zebra event. In one model, although it reasonably describes the conditions in the atmosphere above the active region, the fit of the observed zebra-stripe frequencies could not be made.