sanitizing fresh tanks - basic question

Unless you already have extensive algae growth in the tank, you don't need to shock it. You just need enough to kill what few spores are there. And as the tanks are not subject to sunlight, algae growth shouldn't be a big issue.

Also, if you fill your tank with chlorinated city water, then you already are killing bacteria. It's only if you leave the tank filled for months at a time that you really need to sanitize.