System and methods for customizing fracturing fluids downhole for real-time optimization of micro-fracturing operations are provided. Downhole operating conditions are monitored during a micro-fracturing operation along a portion of a wellbore within a subterranean formation, based on downhole measurements collected by sensors of a downhole formation tester. Injection parameters for a fracturing fluid to be injected from a bulk storage chamber of the downhole formation tester into the subterranean formation are determined based on the downhole operating conditions. Signals for customizing the fracturing fluid using one or more fluid additives stored within corresponding fluid storage chambers of the downhole formation tester are transmitted to a controller of the downhole formation tester, based on the injection parameters. Injection of the customized fracturing fluid from the downhole formation tester into an area of the subterranean formation surrounding the portion of the wellbore is controlled during the micro-fracturing operation.