Your Congo Tetras are a species that gets over 3 inches in length. In the 40 breeder with the Laetacara curviceps and that striped Rapheal Catfish, you are going to be limited in what else you can stock in that small aquarium. If you wanted more fish, you could consider a 1M'4F harem of Apisto cacatuoides. The Central American Rainbow Cichlid, Herotilapia multispinosa you mentioned - if kept as an individual - is a fine choice. Otherwise, a single Keyhole Cichlid, Cleithracara maronii would work. Or, if you wanted to try something a little different, you could stock with a single Dwarf Pike Cichlid. Crenicichla compressiceps is a good choice, as it is probably the most adaptable of the Dwarf Pike species to more alkaline water conditions.
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Crenicichla notopthalmus is a slightly larger-growing Dwarf Pike species, maxing out at over 5 inches in length. But, is also pretty adaptable to harder water conditions and should live peacefully with the fish you already have in the aquarium. I would steer clear of the more sensitive Dwarf Pike Crenicichla regani, unless you can provide a dedicated black water aquarium and can also maintain measured Nitrates in your water below 10-15 PPM.
Crenicichla notopthalmus is a slightly larger-growing Dwarf Pike species, maxing out at over 5 inches in length. But, is also pretty adaptable to harder water conditions and should live peacefully with the fish you already have in the aquarium. I would steer clear of the more sensitive Dwarf Pike Crenicichla regani, unless you can provide a dedicated black water aquarium and can also maintain measured Nitrates in your water below 10-15 PPM.
