It sounded like you meant you'd noticed the darkening over the course of the 2 months you'd had him. Is that not the case? If it's something that happened overnight that's different. Then again I sure wouldn't start tossing in meds if I didn't know what it was. Might be a good idea to post tank size, tankmates, ammonia, nitrite & nitrate reading (with a liquid test tube kit, not strips), water change schedule, etc.
Any kind of veggies will do-zucchini, peas, plants, spirilina, about any kind of greens.