Salty water enema?

So after 2 weeks of trying everything possible there's still hard stool... so it's time for an enema. I don't have an over the counter fleet saline enema... so would plain, warm water be ok to use? Just normal bottled water from the shop, boil until warm, and then use that? Is it safe? The people at the pharmacy/chemist said it could be ok if I just use normal plain water. Do I need to add salt? I Will not be adding soap of course. How much water can I put up there without losing any
"electrolytes"? Or getting bad dehydration or infection? Will adding salt help this? I have an old nozzle/enema tube from 2017 I will obviously clean really well/sterilise. I'm nervous about doing just plain water since Google says it's dangerous and water intoxication or low sodium levels. Would adding salt help? Is it dangerous to do it without salt?
PLEASE HELP! I haven’t left the house in a MONTH because of this!