How to stay healthy when drinking lots water

Hi, i have a challenge me and my sister started a while ago of drinking a gallon of water everyday (gallon a day May) for a month because we already drank lots so we decided to make it a month of a gallon instead. This became a regular thing which alternated months.

During this, i started to pick one day to challenge myself to drink as much as possible in a day. The number has climbed and the latest was 16 litres in 24 hours. I want to reach 18 this time so an even 4 gallons.

I understand more now than i used to the risk of drinking lots of water and i wanted to know the best way to stay healthy during. I know electrolytes are supposed to help but is just some natural salt going to do much?