Pumps lower the risk by keeping the temp even throughout the system and keeping the water moving so there are not dead spots.
You can cool the buckets with ice, a chiller or whatever, but how are you going to cool the water sitting in the tubes unless you're like the guy growing in the basement?
You can cool the buckets with ice, a chiller or whatever, but how are you going to cool the water sitting in the tubes unless you're like the guy growing in the basement?