Need to replace seal between bowl and cistern.
But is there a way to stop water supply to just that one toilet, it has a tap thing next to it, see pic. Tried turning it, but no luck should I turn it harder?
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Before trying to move a stuck shut off valve, determine how to shut off the water to your home or apartment. This is the main shut off valve. It also may be stuck, so you might want to try shutting off the water before proceeding.
To free a stuck shut off valve, first try opening the valve a little more, then try closing it. The back-and-forth motion may loosen the mineral build up sticking the valve.
Next, try a few short squirts of WD-40. WD-40 is a penetrating solvent. It will wick into the spongy mineral build up. You may have to remove the valve's handle to see the valve stem. Spray the WD-40 where the valve stem (shaft, like a pencil) enters the valve body. A little goes a long way here. A short spritz, followed by a 2-3 minute wait, then another spritz and wait should free up the valve.
Before resorting to pliers and other strong measures, shut off the main water valve and drain the pressure in your water system. The more force you use, the more likely you are to break the valve.