Hello Msquare
Thanks for your much valued reply. I shall wait a bit to see what other experienced users like your good-selves might suggest for my question
With regards to communication after alarm triggering, it's just a matter of sending an output from the Arduino to energise a small 5 volt relay whose contacts trigger the home alarm just as any other zone sensor device
In case of which method of sensing to use for the water level, the column of water I need to measure is approximately 90 cm and the tank's width is only 60 cm. The distance between the maximum water level when the tank is full and the top cover of the tank is just 5 cm. The flex sensor you mentioned is a good idea but I won't be able to have a full scale reading of the water levels. Besides, the flex sensor is also a delicate and a bit costly device compared to other type of sensors
I shall try asking in the General Electronics forum here as you suggested
Thank you once again for your replies 