Reptiles/Chinese water dragon not eating
I have a chinese water dragon who's about 1 year old; I've had him since July. He/she (not sure which) ate and shed constantly until about 2 months ago. He still eats, but not nearly as much as he used to, seems to weigh less (just going by feel here) and hasn't shed at all lately either. He is not noticeably thin, but he was more robust a month ago.
He's still active too, though maybe not quite as much as he used to be. He still drinks water, and i'm not sure if i can distinguish whether or not he has diarrhea (didn't look like it to me at least)
His hot spot is just over 90F. Humidity, admittedly, has been hard to keep up and usually hovers around 50%. His swimming area has about 6 gallons of water and is filtered. The substrate i had been using was cypress moss, but I recently switched to moss.
Any idea what could be the problem? Or if there is indeed a problem at all?
He has lost a considerable amount of body condition in just a few months--I would say that he is probably quite ill. His body condition now is verging on emaciation. My recommendation is to take him to an experienced reptile veterinarian right away, along with a fresh fecal sample. (You may be able to get one out of the water with a small fish net, or turkey baster). There are a number of possibilities, but parasites is certainly one of them. They can build up to lethal levels in captivity, due to reinfection in the enclosed environment.
I don't recommend using filtration in water areas for these animals, it simply isn't enough. Instead, consider replacing his water area with a plastic cat pan which can be lifted out. These animals almost invariably defecate and urinate ONLY in their water, which means it should be changed completely and scrubbed out every single day. (Some people do so twice a day). I can see that you have a nice naturalistic vivarium, but unfortunately, when it comes to water, cleanliness is far more important. Dirty water can quickly lead to infections.
He needs very prompt attention at this point to diagnose the issue. Sadly, if he tests positive for parasites, you will have to tear down the entire viv. Everything will have to be cleaned and disinfected, and discarded if it can't be disinfected. He'll need to be kept in a minimalistic enclosure during treatment, and kept scrupulously clean.
However, given his condition, you will need to act immediately, or risk losing him.
You did not mention UVB lighting. Water dragons require this, and if you're using fluorescent UVB lights, they must be within 8 inches of the animal to be effective, and must be replaced every 6 months. The new Mercury-Vapor reptiles lights last for 3 years, and double as heat lamps--they throw UVB much further, and may thus be more economical in the long run, though they are more expensive. They produce quite a bit of heat, though, and are suited mainly for larger enclosures--for a water dragon, that should be fine, as these animals grow large.
Food should be offered every other day, as much as the dragon can eat in a single sitting (remove any leftovers after 15 minutes). Insects should be dusted with calcium, and gut-loaded. Dragons may be offered crickets, mealworms (when small), zophobas worms (when larger), feeder roaches, nightcrawlers (large earthworms sold for fishing), silk worms, phoenix worms, frozen/thawed baby mice or rats, and strips of fresh tilapia fish (never frozen, as freezing creates thiaminase). At age 1, it's time to start introducing greens. Some water dragons will eat greens heartily like an iguana, others only nibble on occasional bits. They're fond of small pieces of fruit and colorful edible flowers.
With a water pool deep enough to submerge in, a humidity of 50% is no great concern. Good air flow will help prevent little-understood maladies such as tail rot. I see his snout is undamaged, so you have avoided the face-rubbing issue, which is great.