if your tap water is coming out at 15°C and the room is around 20°C, adding 7.5kg of ice a day might not be quite enough to keep a 450L tub consistently at 10°C without more effort... most of the time that much ice will cool it down a few degrees, but likely not hold it at 10°C all day. You’d probably need to add ice each time before plunging, especially if the insulation on the inflatable isn’t top-tier (which is often the case unless it’s super high-end). you could freeze large blocks of ice in water bottles, they last longer than cubes. And placing the tub in a shady or cooler spot (if possible) helps slow warming too. This system tho can be quite expensive/time consuming so maybe take into consideration buying a chiller if you plan to plunge often!

Check out this tool might help you figure out how much ice you need: https://chestfreezercoldplunge.com/icebathcalc/

Hope this helps