So this is something people tend to forget about when they try and do full hydroponics with stuff sitting in water. Yes 24° is too high, you need to bring that down to about 20°.
The proper solution to the problem is a water chiller, however...
500ml or 1L coke bottles filled with water and put in the freezer also works.
You do a bunch of them and keep them in your freezer, then just drop 1 or 2 into the reservoir and it will bring down the temps, they will last most of the day and you can just cycle them.