I have a 20gal tank as well and have been carring up to 4 doz lately and have had no problem keeping them good all day.
This time of year I fill it up and put in a little defoamer and 3/4 bag of ice from the get go and add the other 1/4 within an hour and good to go with 0 problems.
I have been getting mine at the Oakwood bait store.
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Eat, Sleep, Hunt and Fish (The Devil is in the details)
Dan Duncan