You CAN in theory have a single 3 inch specimen in a 29 gallon tank in freshwater, and you have a very balanced aquarium, never having to do water changes. But no one does it...
ALSO freshwater can be ridiculously expensive too. Just look at high tech planted tanks! A CO2 Diffuser is like 200 bucks at least, plus extra lighting and fertilizers, not to mention buying all those different kinds of plants. AND People often pick discus and cardinal tetras. Discus are at least $50, sometimes $99 even. Cardinal tetra are often wild caught and 50 $2 fish adds up...
You can heavy stock a saltwater tank if you have just fishes and macro algae. Same goes for freshwater. They aren't that much different.
PS: I have seen tobys in the wild... at about the size of the OP's. Do they ever get big?