Cold water is key along with fresh water, tap water is probably not the best choice due to added chemicals, assuming you have city water. Distilled, spring water, or well water being better choices for refreshign their water. I keep them in the fridge, and make sure to keep the cold as much as possible when I use them out fishing. I’ve heard, (although I cannot back it up with fact), that once the water temp rises above a certain point the leeches go into breeding mode, and will die shortly afterwards. Basically cold fresh water, and if you can slide it past the wife’s eyes hide them in the fridge. You could keep the in really any container assuming you follow those 2 hints., but you might want to be careful if put them in a gallon milk container than put them in the fridge, or a divorce could be short comming
That or you can use my backup method to keeping leeches alive, and not catch and fish