More specific measurements of digestibility now seem to be replacing the use of Cr2O3 as an indicator. Measurements of the caloric values of ingested food and faeces produced provides some part of the information for estimating the energy balance for a fish (oxygen consumption and growth rate are also needed). Alternatively, protein (measured as nitrogen content) or lipid content of food and faeces could be measured. In both cases the equation would be:
In measuring protein nitrogen, one really should take the nitrogen from the deaminated amino acids into account, which requires measuring the ammonia excreted by the gills (metabolic nitrogen). That equation should be: