A month ago i took my car to fix a little leak of coolant in the radiator, this mechanic fill it with simply water, until see if the leak was covered, well during the first 3 days the car leak a lot of water when i arrived home after work, then it stopped, i think this is because of overfilling right?

Well now i'm sure it is not leaking i'll refill with the correct mix of water and coolant, but it is working so great, the temp is so regular, that i feel nervous about changing it, i know water is so corrosive but every time i mess with the radiator problems begin...

How many days will it leak if i overfill?