The short answer
Red yeast rice can lower LDL, but the dose and purity vary. Over-the-counter fish oil doesn't lower LDL.
Red yeast rice contains the same chemical as the statin lovastatin, without the quality control of a medicine, so it can have the same side effects.
Fish oil lowers triglycerides but leaves LDL alone. A prescription form (icosapent ethyl) is used for some people with high triglycerides.
Source: US FDA warnings on red yeast rice; REDUCE-IT trial, NEJM 2019.