Have to agree with what scott is saying here.
I personally don't see a problem with 70% daily water changes provided the new water matches the old water chemistry wise, and that the large water changes aren't stressing the fish. Atractosteus genus gars can be sensitive to pH fluctuations.
Edit: Forgot to add that most of the live bacteria in a fish tank is on the hard surfaces in the aquarium, not in the water column.
Each gar is different in how dark or light the base colouration is, but this is still influenced by environment. Some don't lighten or darken as much as others.
For example in my florida gar pond, there are 20ish floridas all displaying rather dark colouration from the black background and sides. When a new gar is added it usually displays a lighter colour than the current floridas but will darken over the next couple weeks. There is still individual variations amongst the floridas with some being lighter or darker yet all are darker than when they were kept in a lighter tank.
Have you considered adding something like a sponge filter to the tank? This will help a bit with the water quality issue by adding some biological filtration. Also I would suggest removing uneaten foods after the gar has stopped eating.
