stang, feeding, fertilizing, aeration, choosing fish species that feed on otherwise unutilized nutrients (ie. tilapia eating FA, Golden shiners filter feeding) and in turn become optimally sized prey fish would be bound to increase carrying capacity. It can be extremely difficult to say what impact one forage species will have on the rest of your pond's ecosystem. There are almost always unintended consequences from everything done in a pond.

Crawfish and shrimp can be great additions, but I think I would personally avoid any leeches (though most ponds have them appear sooner or later naturally). Clams and mussels may over-populate and keep your water TOO clear, which reduces micro-organisms in a pond. Nature will always balance out, but it may or may not be a balance you want....