I am looking for a solution to prevent a water tank standing on ground level, from draining on gravity pressure through the pump system when the pump is turned off. (no electricity feed I mean)

After the pump there is no manual valve or water tap.
So that is why the tank can drain.

A pump (this pump) does not close the water flow from tank to outlet of pump when turned off (electricity).
I think most pumps, if not all , don't.

After the pump I do not want a manual valve or water tap.
It is for a garden sprinkler system so the outlets should always be open.

Of course I could work with a solenoid valve on a timer, but I rather put the PUMP on a timer to save money.
This way I do not need a solenoid valve, a pressure switch, a pressure tank and a safety pressure relief valve.

But if the pump is not turned on , the water from the tank slowly flows through the pump on it's gravity pressure, causing the tank to drain through the outlet of the pump.

I was thinking to install in reverse position a check valve with a spring after the outlet of the pump, but the spring of the check valve is not strong enough to stop the water from opening the valve.

If the spring would be strong enough, the gravity pressure would not be high enough to push the valve open, but when the pump turns on, the pressure of the pump would be high enough to push the valve open.

Or is there a more standard solution.....