| | Re: Question about water pumps.
It should work but there a number of things to consider, will the water we able to be cooled quickly enough while it travels through the fridge to your desired temperature? you could increase the cooling by having coils of tubing to increase surface area for cooling, but the pump needs to be able to cope with this.
Is your pump going to be able (strong enough) to pump the water through the fridge and back up out into the tank again? is the pump you plan on using your filter or a power head?
If your heat waves are only short you could just go with the ice bottles on rotation to cool your tank, it could be alot less effort.
Just give it a try and see how it turns out DIY is always plenty of fun