With modern filtration and regular water testing, the old stocking levels quoted by many sources are now a little obsolete. Variables such as size and temperament of fish are factors to be born in mind and all systems will benefit from slow and cautious stocking. Remember that most aquarium fishes are highly social and need to be kept in shoals in order to thrive. Rather than create a ‘Noah’s Ark’ aquarium where fish are stocked two by two, choose your favourites and enjoy the sight of them as a shoal.
Due to the large size and messy nature of goldfish, I would limit stocking to just two fishes that would quickly grow to a size that might make a 60cm aquarium look small. Bear in mind that although the fishes will grow, the tank cannot, so always calculate stocking levels based on adult sizes of the fishes you choose. As the waste levels in an aquarium are linked to stocking densities, a heavily stocked aquarium will require more maintenance than an uncrowded set up. Tropical and subtropical fish communities normally feature small fishes often averaging a little over an inch in length and these are the species of fish most suitable for the smaller aquarium.
Using an old formula of 12” of fish for every square foot of surface area, an Aqua Tropic 60 comes out with a carrying capacity of approximately 24” or 60cm of fish length. Obviously two 30 cm fish would produce a massive amount of waste in comparison to twenty fish 3cm long, so here we see the limitations of this method. A more modern stocking level of 1 cm per litre gives a not dissimilar figure of 65 cm of fish and this can be slightly exceeded with efficient filtration and regular waterchanges. As a guide, twenty small shoaling fishes such as tetras or white cloud mountain minnows will leave scope for a few catfish, shrimps or one or two slightly larger ‘feature fish’.