If the rent-to-own has been completed and you now own the home, you can sue the seller IF you believe you can show that he knew, or under the circumstances must necessarily have known, about the undisclosed septic tank: a failure to disclose a known issue is fraud, which would provide a basis to sue and seek compensation. But as stated, the seller must have known: if the seller did not know, the seller did nothing wrong, since you can't be held liable for not disclosing what you were unaware of.If you don't own yet and are still a tenant, the landlord (since until you actual finish buying the house, the "seller" is actually your landlord) has to correct the odor situation, since it affects habitability; if he doesn't take steps to correct it after you provide written notice, you could withhold rent to force him to remediate the situation. If you do this, be sure to hold onto the withheld rent and have it available if the landlord takes you to court.