Wa Alaikum Salam. Bro, never use tap water, it will rust all of the cooling system of engine, and, you should pour the new one after complete flushing of the old Coolant.
You can also consider Toyota Genuine Long Life Coolant. As per details available on http://www.toyota-indus.com/after-sales/parts/chemicals/ regarding Coolant, there are 02 types of coolants (one is 50/50 premixed, the other is 100% concentrated and needs to be mixed with an equal amount of 100% pure water, to make it 50/50).
Let me quote complete and exact details to make it more elaborative and clear:
"... Genuine Toyota Super Long-Life Antifreeze/Coolant is the same high quality, factory-fill product found in new Toyota Vehicles. You’ll know it by its distinctive pink color.
It provides maximum protection and durability without the use of harmful silicates. And because it’s compatible with non-metallic materials, it helps to extend the life of water-pump seals.
Don’t trust your engine to anything less than the best: Genuine Toyota Super Long-Life Antifreeze/Coolant.
Product Range
Toyota Genuine Long Life Coolant
(Pre Mixed, Anti Rust & Anti Freeze & 100% concentrated)
Toyota Genuine Super Long Life Coolant
(50/50 Pre-Mixed, Anti Rust & Anti Freeze)
General Characteristics
Provide excellent cooling system
Provide freezing protection down to -37ºC & boiling protection to 108 ºC
Prevent engine to freeze in low temperature
Entirely safe to use in aluminum components
Rust prevention
Decrease corrosion.
More than 40000 KM mileage or 24 months
* 50/50 pre mixed Coolant do not need to mix with water.
* 100 % concentrated Coolant is pure & need to be mixed with water... "
