Keg King highly advises using homebrew-grade cleaning chemicals like phosphoric acid-based sanitisers and non-caustic cleaners. Follow the recommended dilution rates on the product labels or from reputable homebrew books and magazines.
Phosphoric acid sanitisers should have the pH range between 2.2 - 2.8. This is about a 1.5 - 2ml of sanitiser in 1L of water. Above the pH of 2.8, the sanitiser fails the 30-second test to prevent microbial growth. Below 2.2ph range, the sanitiser will affect the flavour of beer.
Heat does more damage to PET fermenters than chemicals. At 55 degrees Celsius, the PET will soften and deform permanently. It is best to allow hot cleaning solutions to cool down to at least 30 degrees C before use.
Adhering to these recommendations will ensure the integrity of your fermenter tank for its useable life.