How much plain water should a 15 months old be drinking daily ?

I don't think there is a guide to how much water a kid should be taking. You have to monitor his pee and poo to know whether he is getting enough.

I will gauge usually by his activities. If he is very active and running, i will give him more water to replenish his lost of water

My daughter loves water. She drinks any amount of water we give her. If her activities are more, we'll usually give more water.

There is no guide. If the weather is very hot, I would give more water. I also give water after every meal.

at least 1.5 litres for detox

I would say it’s depend.