Fixed wireless systems are designed to provide broadband connectivity to residential and enterprise customers. Since they typically operate in frequency-selective low-Doppler conditions, a preferred signalling choice is OFDM (orthogonal frequency-division multiplexing). In contrast with mobile systems, where the channel variations are often too rapid to be followed by the transmitter, in fixed wireless such variations can be accurately tracked. This provides the added flexibility of allocating power over the OFDM tones. For ideal Gaussian signals, the allocation of power over parallel channels is solved by the classical waterfilling policy. For the discrete constellations used in practice, however, waterfilling is no longer optimal. Rather, the power allocation that maximizes the mutual information is then given by the more general mercury/waterfilling policy. This paper illustrates the usance of mercury/waterfilling on frequency-selective OFDM channels with QAM constellations and it quantifies the extend to which conventional waterfilling curtails the mutual information.