Toilets are notorious for being difficult to keep clean. The problem usually stems from bacteria that builds up inside the rim of the bowl, which is difficult to reach. To clean the inside surface of the water flow path you’ll need to get creative.
Start by shutting off the water supply to the tank. Flush the toilet to empty out all the water. Dry off the underside of the rim and apply duct tape to block the tiny siphon holes. Fill the tank up with a gallon of pure vinegar and flush it down. Open up the water supply to let the tank fill back up.
The duct tape should hold the vinegar inside the rim. Let it set overnight so the vinegar can fully kill off any bacteria. In the morning remove the tape and scrub the siphon jets with a nylon brush. Flush the toilet to wash away any debris. This should keep your toilet clean much longer!