Many, many decades ago when I worked in a restaurant, the rule was tipped employees needed to be paid at least 50% of the standard minimum wage. At the end of the day, if the employee didn't clear the minimum wage rate with the total of their wage + tips, the employer was required to fork over the additional amount to equal the minimum wage.

However, the absolute bare minimum the employer was allowed to pay was 50% of minimum wage. Not sure if this was just state-specific or if it changed.