Go to Canadian tire and get a couple of clip on fans to clip on the side of the tank.
If you can't use fans, thern you are in for a lot of manual labour - freezing bottles of water and dropping them in the tank, then removing them and refreezing them and putting them back in the tank.
Or you could spend outrageous money on a whole bunch of peltier devices...
Beyond that, your options are really simple - get a chiller, or use more fans...
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