Planted Tank Obsessed
Join Date: May 2015
Location: Central Michigan
The main reason fish need the space in a home aquarium is largely based on bioload. If you had 10 tetras (example) in a 10 gallon but sumped to a 100g tank, those fish are now in 110g of water regardless of only having 10 gallons of space.
Obviously confined space is still an issue but you could keep a 10" inch goldfish in the example (sumped 10g) tank above as far as water quality is concerned. Obviously there is still things wrong with this example but your water quality wouldn't be a concern.
The shop guy didn't necessarily give you false information, just not all of it. Yes, as far as fish that don't get that big (<1.5") you could get away with conservatively overstocking in a well established heavily planted tank (regarding waste) and 10 gallons is as small as I would go if I were to do that. But like previously mentioned, your tank doesn't just get bigger (water volume) because of plants, it's much the opposite.
Above all else, the tank owner can have a tank as healthy or sick as they want with any fish, just comes down to properly maintaining such a small space, which can become a full-time job in and of itself.