Don't use acid on any ceramic fixture. If your fixtures are staining try CLR or replace them. CLR works good on most stains, but if the fixture has lost its glazing, there is nothing you can do. Think about the cost of replacing the fixture and how long it will last. Most fixtures have been in homes for 30 to 40 years. The cost of the original fixture was minimal. In 1972 a toilet cost $27.00 and a car was $2,000.00. Today a toilet costs $95.00 and a car costs $20,000. Yet you replace the car in 7 years. A toilet holds its value much longer at a lower cost. A toilet has only gone up 4 times in value, and a car has gone up 10 times.