Overstocked, but in terms of fish species compatibility, not numbers. I am assuming this is a standard 30-inch 30 gallon? That in my view is not sufficient space for more than one dwarf cichlid species. Mikrogeophagus altispinosus
] attains 4 inches and should be kept singly or as a male/female pair in anything under a 3 foot tank, and safer in a 4-foot. The common Blue Ram
] is pushing it with more than a pair, but with lots of wood and plants to delineate territories it could perhaps work. Apistogramma species can work with a pair or a harem (one male, 2-3 females) depending upon species, as some work better one way or the other. You can see more info in our profiles, click on the shaded fish names. One apisto is included so far, Apistogramma cacatuoides
, and most of the data will apply to most other species.
I think the filter is too powerful for these fish, and especially with plants. All of these cichlids occur in very still waters, usually pools, lagoons, slow-flowing streams, and flooded forest. You might want to consider a sponge filter, much less expensive and good filtration for the fish and plants.