1) IF this is the case, PLEASE follow this guy's intructions
2) IF you mean that you change 1G of water/week then the problem isn't as drastic, but still very serious.
EDIT: Nevermind, I just read that you put 1/2 spoon of salt into the ONE GALLON OF WATER.
I feel so sorry for this frog, how it lived three years perplexes me. It looks like you've done some very basic research, how could you exclude something so so basic when it comes to frog-keeping? I can't see any human being thinking this would be okay for a frog, seriously, I bet the frog can hardly move in a 4L environment...