But it would add up to about 15 gallons a month . They always say 20% water changes ?
It wouldn't be quite that much, because you're also removing some of the the previous days' clean water additions. But in this case, it would be very close to a total of 15 gallons changed out.
Basically, you are correct - Frequent small water changes are nearly as effective as less frequent large ones. Also much smoother for livestock and less likely to cause abrupt changes in water chemistry. Also much easier to do for some people. I personally don't do any changes of more than 5 gallons in either of my 100 gallon systems, I just do more of them if needed.
RHF has discussed this in one of his articles.
Edit: Here's the calculation for your tank:
0.5 gallons/80 gallons = 0.00625 = 0.625% This is the percent of total water you're removing with each small water change.
1 - 0.00625 = 0.99375 = 99.375% This is the percentage of 'old' water you're removing with each small change.
1 - (0.99375)^30 = 0.172 = 17.2% This is how much old water you've removed after doing 30 daily half gallon water changes. This is only slightly less than you would guess based on 0.5 x 30 / 80 = 18.75%.
Also, the logical end result of doing smaller more frequent water changes is continuous water changes. This is a fairly popular thing, doable with the Neptune DOS and probably other systems as well.
