I also have one common gold fish, and I also had a 5 gal hex tank in the beginning. I ended up buying 46 gal used tank with a stand for $140 from Craiglist website. Now my water changes came down to twice a week untill my tank is fully cycled.
I agree with everybody, please keep doing the water changes every day untill you can have at least 20 gal per fish. Your fish does look like a comet, needs a lot of water.
As I know by now that high nitrates can also cause the red marks. If after daily the water changes the nitrates went down to 5 ppm then what was it before? They say that it is safe up to 40 ppm, but I know the cases when nitrates above 10 ppm caused fish to get the red marks, stress and sickness.
What percentage of water do you change every day? I did from 50% to 85% in 5 gal, and could barely keep up with the rising ammonia and nitrites, especially when the water temp was higher than 74. With you temp 78 the ammonia, nitrites, and nitrates are rising much faster.