FWIW the original works as well, but because you're using a typedef (not an alias) you have to convert from your own type to the underlying:

    func (t *fishTank) fishCount() string {
        return fmt.Sprintf("How many fishies? %d!", (*container[fish])(t).size())
    }

that's because

    type fishTank container[fish]

creates a completely new and independent type with container[fish] as the underlying implementation.

An alternative is to just alias:

    type fishTank = container[fish]

however in that case you can't define methods on fishTank, because it's literally just a shorthand for container[fish].