BTW easy to figure out if you want to go the cheap way, how much you need. Just start with a two liter bottle in your 30 gal res and test it. Put solution in there and leave it 24 hours so it heats up, then put your two liter bottle in. Check the res in a couple hours and see what the temps are. Check it a few times and see how long it keeps it cool. You will be able to figure out a rough idea of what you need. I just put the liter bottles in, in the morning, and before I go to bed every day.
Have fun! It's a labor of love man.