depends on stock. just the gar in the 125g. sounds good maybe even a little less. just the gar in the 55g work for awhile and 25% should be fine. Im not the best on this sort of thing......lol
i always use well water, and i think the 125 will be fine, it is wild caught it had way more than 125 gallons every day of its life till you caught it, also you can dechlorinate by leaving the water for one day like in a jug i believe?
this is not the case with a captive fish, even one captured from the wild. with a gar that small a smaller tank is necessary. just because it had more space in the wild doesn't mean it will do well in the similar situation in captivity, especially with a young fish. in the wild that larger area is saturated with food, and young of the year gars actually occupy a very small area for quite some time in their first year.
start the fish off (if it's 6") in a 20g long or 50g breeder. a 55g will work for a couple months as well; but move it to the bigger tank once it gets to about 10"-11" long.--
--solomon