Thanks all for the recommendations.

What I need is a method to monitor the water level in the tank. When the tank gets empty, the key chain will go up. When full, down. The red tape is the full and empty indicator. There is a ball in it stop the water level when full. Only when the water level is empty, I need to cut it out in order to protect the hitachi pump.

One solution I found was this https://paremtech.com/ kind of level. Automatic sends notification to phone.

At first I was also confuse .... water, light, off pump.

The reserve water tank is to store water from the main and to be pump to the various part of the house.

In most houses, water from the main is faster or equal to uage but not in ts case .....

Incoming flow from the main is slower than usage ... hence need to turn off the pump b4 the water dries up to prevent the pump from damage.

The core idea is the tank is used as feed tank. Feed tank must not run dry. If got water then it is safe to run pump.

Not used as filler tank so we cannot use fill control type for aquarium that stops when at Hi level.

My house area water pressure from main is slow. Very slow. Weekday from 10 am - 10 pm no water. Hence there is a table next to the main pipe for catchments from 10 pm onwards. There is another tank at back of the house. Between the first and second tank, there is a pump I cannot let the pump run all the time as sometimes the water from the front tank gets empty. So I need a mechanism to cure off water when it is empty

What kind do you suggest?

Alternatively, if anyone knows where I can find a high capacity low profile tank, that will work too.

yes but on hold for the moment.
Correct but my setup, the water for mains has to travel like 3 m and climb 1.2m to the first tank. Even this one also tak larat-shows how poor the water pressure is here.

For now settled on a simpler solution. Put smaller tank 100 gal below main water level, and wait. Since my last tank on roof use float valve only push 50 gallon per time, this should give enough time for the small tank next to the mains to recover the water sent. Will also settle the problem of sensor. As all other tanks use pressure sensor based on the float sensor on the roof tank.