First of all, we have to agree on the volume difference, but I'm assuming my calculations are correct because I am an engineering student and I've done this type of calculation hundreds of times. If you disagree, explain how it's wrong and correct me.
Ok, so the actual numerical value is:
Liquid has 3235 times the heat capacity of air, given our parameters.
Now you do have a valid point about air being easier to move than water, but let's quantify that difference.
Let's say that our liquid cooled set up uses a 400GPH pump.
400 gallons per hour = 6.67 gallons per minute.
1 gal = 0.13 cubic feet
6.7 gallons * 0.13 cubic feet = 0.87 cubic feet per minute
To determine the equivalent fan size needed to match the heat capacity of that amount of water, let's find out how many cubic feet per minute we need:
3235*0.87 = 2814 Cubic feet per minute.
Good luck finding a couple of PC fans that can produce 2814 CFM for $40....shit you won't even find something that powerful for $400.....
Now can you understand the difficulty in having your cool tube surface stay at 85F? Especially with a 600W