Its weird there is no product to resolve this issue.
There are several commercialy made chillers made just for cooling aquariums.There are even directions for DIY chillers (see below)
1. Locate a WORKING dorm-size fridge
2. Buy about 100' of 1/2" plastic hose from a hardware store
3. Coil the hose in the fridge
4. Drill 2 holes in the top or side of the fridge that are just large enough to fit the hose(avoiding the internal coolant lines)
5. Feed one end of the hose out through a hole over to the sump
6. Feed the other end of the hose out through the other hole over to the sump
7. Attach either hose end to a submersible pump
8. Place the submersible pump in the sump
9. Attach an inline valve to the other hose end and also run it into the sump
10. Turn on the fridge and allow to cool down
11. Record the tank's temperature and plug in the submersible pump
12. If everything went well, you will have a running loop of water from the sump to the fridge and back again with no leaks
13. Check the temperature often and make adjustments as needed
You will have to play around with the flow rate using the valve to control the temperature. Slower the flow, cooler the water. Other variables include hose length, hose thickness, tank temperature, etc. The length of hose in the fridge my need to be lengthened if the temp doesn't reach your target number. The opposite might also occur, although this could fixed merely by raising the fridge temperature.
Once the hose is where it needs to be in terms of length, then the holes in the fridge should be sealed with silicone sealant.
A good safety feature that you may want to incorporate would be an electronic thermostat. You would place the sensor into the tank, set the unit's low temp point no lower than the fish could handle, plug the pump into it and plug it into the outlet. This way if the tank temp gets too cold, it shuts the chiller loop off.
If something goes wrong and you have a catastrophic failure, I take no blame
