I disagree. Just because it would be difficult doesn't mean it is impossible. You would need the right balance of prey to predator. For example, a 125 gallon tank heavily planted fully populated with cherry shrimp and guppies (and microorganisms, invertebrates, infusoria, etc)... would all these populations crash if you added, let's say, one golden wonder killifish? I doubt it. Somewhere between that and too many/large predators is the right balance. The eventual complication with the rams is that they will breed and when their population gets large enough they might decimate the shrimps and whatnot. You would need a predator that would eat the ram's fry to control their population, but not decimate everything else. Of course it is complicated, but not impossible.