Activated carbon is a short term remedy.
I'm not saying that granular activated carbon (GAC) is a fix-all remedy, but it is a valuable and inexpensive tool to have at one's disposal. The same goes for the various zeolite and other synthetic resin blends available. They won't fix the root cause of a tank malady, but they'll alleviate the symptoms. I would tend to disagree that is isn't worth the cost. A cost/benefit analysis would be up to the user. In this case, where repeated water changes are providing the desired effect, it certainly could be worth it as a "polishing" remedy - especially if "crystal clear" water is what the user if after.
It will last around 2 weeks in most cases and after that whatever was the cause of the discoloration will begin to reappear.
That first part isn't true, at least unless your water is really nasty. The lifespan of GAC depends on the concentration of contaminants and what the adsorption rate for those contaminants is at a given temperature. This can only be determined via an emperically-derived isotherm. Luckily, these are available for the most common conaminants. In most cases, the usable life of GAC is much greater than the manufacturer will suggest (for aquarium-grade GAC, not environmental grade). I do agree that once the GAC is spent, the symptoms will reappear. Note that GAC can be re-activated after it becomes spent by cooking it at a high temperature. Re-activated GAC doesn't last as long as "fresh" GAC, but it is a cost-saving alternative to throwing it away. Depending on the quality of the GAC and the efficiency at which it is re-activated, I'd expect a 30-50% loss in capacity after re-activation. But still...
Also note, GAC does NOT adsorb all contaminants. Phosphates and nitrogenous wastes, for example, require other synthetic resins (or, as Boomsloth and I both advocate - more water changes).
To the OP, what do you feed your axolotls? If they're feeding on earthworms, chances are that the gut contents of the worms is being emptied into your tank water. This may be happening directly (the worm poops) or secondarily (the axolotl which ate the worm poops). Many commercially available worms are fed peat moss or organic compost, both of which will color your water yellowish. This is on top of the coloring effects already inherent in feces, decaying leaves, etc.
Regardless, either increase your water changes or add a filter medium that will remove the conaminants from the water (or do both). Putting a bigger filter on the tank isn't going to fix the problem.
-Cole
