Carbonate budgets describe the balance between the production and erosion of reef calcium carbonate, and underpin crucial ecological functions and ecosystem services. Carbonate budget models can fill a need for holistic reef monitoring tools, which are necessary to support effective reef management decisions in response to the threat of climate change. On the Great Barrier Reef, carbonate bioerosion is primarily driven by parrotfish grazing, but our understanding of this process is mostly limited to shallow reef environments. In order to better understand how bioerosion and carbonate budgets change with depth, we investigated the feeding behaviour of five species of parrotfish along a reef slope (2-11m) at Heron Island. We used visual observations and video recordings to explore how parrotfish feeding varies according to characteristics of the benthos, including substrate density and complexity. Substrate density is a particularly important factor in calculating the contribution of bioerosion to carbonate budgets. Our data reveal the complexities in disentangling parrotfish feeding patterns across species, body size, depth and other environmental variables, often with nuanced outcomes for bioerosion. Our data help to add ecological realism to carbonate budget models, improve our mechanistic understanding of reef functioning, and the development of informed monitoring tools.