MeerFish implements a Fisher forecasting framework tailored to upcoming large-sky single-dish HI intensity mapping surveys such as those planned with the MeerKAT telescope and its successor within the SKAO project. The code models survey specifications and expected statistical uncertainties to compute Fisher matrices and forecast parameter constraints on cosmological observables derived from 21 cm intensity maps. It provides modules to define survey configurations, cosmological and nuisance parameters, and the associated signal and noise models needed for forecasting survey performance. MeerFish outputs predicted errors on power spectrum measurements and derived parameters, enabling quantitative assessment of survey design choices and scientific reach.