Conn. Gen. Stat. § 12-419a

In the case of any tax which is subject to penalty or interest, or both, pursuant to section 12-414 , 12-415 , 12-416 or 12-419 , which is outstanding on July 1, 1990, and which is not the subject of a payment schedule or settlement between the Commissioner of Revenue Services and the taxpayer, the taxpayer shall be required to pay a penalty on the amount of such tax outstanding on said date in the amount of five per cent and shall be subject to interest at the rate of one and two-thirds per cent per month or fraction thereof for each month or portion thereof commencing on or after July 1, 1990.