From what it sounds like to me, the problems you are experiencing are caused by three things:

1. Glass enclosures have very high thermal conductivity which causes high heat loss, and loss of humidity. Depending on your ambient room temps, with the high heat loss of the glass, the glass remains cooler than the internal tank temps causing condensation to  form on the inside of the glass drying out the air. In this case, the glass walls act like a dehumidifyer. Many people often mistake fog on the glass for adequet humidity.  Also, most glass enclosures use some sort of a screen top which causes a chimney effect for heat and humidity to rise from the enclosure. All of these issues make glass cages difficult to maitain proper temps, and almost impossible to keep proper humidity.

2. Lack of proper heating. In such a large enclosure, with low efficiency, a simple under tank pad will not adequetly heat the enclosure. JCP's will like to climb, therfor, the tank should have a warm and a cool side. Your current set-up is only providing a warm spot on the floor for the animal to warm itself on. I would suggest using a RADIANT HEAT PANEL such as a Pro Products RHP. ( Pro-Products.com) . RHP's mimic the heat rays from the sun providing a gentle warming effect for the animal to bask under.

3. Poor ambient room temperatures. With your large,  in-efficient enclosure, with inadequet heating, your entire set-up is struggling to provide proper internal temps. If you are attempting to create an enclosure with say 86-88f on the warm side, you will have to boost your room temps to somewhere near 80f. Most reptile set-ups will only compensate for 10-15 degree drops in temps with proper heating. With more than this, you would have to use too large of a heating element effectivly "cooking" your herp just to maintain adequet heat.

My suggestions....

1.   Leave the fish tank to what it is best suited for....FISH. There are plenty of decent manufactures who make much higher efficency front opening enclosures. A few choices may be PVCCAGES.COM , CONSTRICTORS NORTHWEST, BOAPHILE, HERPCAGES.COM, or any other manufacturer using some sort of plastic for construction. If buying a cage isn't an option, one couls be built from wood or melamine for relatively cheap.

2. Proper heat such as a RHP. Once you find a proper enclosure, email Bob a ProProducts, he will help you match up your enclosure to a proper size panel. ALSO, remember to always use a thermostat with any heating device. I reccomend Herpstat thermostats from spyderrobotics.com , but any decent thermostat should be fine.

3. Maintain your room temps to around 78-80f.With these temps, even the largest of enclosures will hold decent cool side temps, and allow a PROPERLY sized heating element to gently warm the warm side of the enclosure to proper temps.

If you have any other questions, or need any clarifications, feel free to message me.

                     Seth Patrick

An easy fix to improve humidity- Place a heat source over the water dish, and place a towel over the other end of the tank. This will cause the water to evaporate faster, and the towel with help trap some of that moisture in the tank improving the overall humidity level.

Happy Handling!

i have asimilar set-up for my large male carpet.  i covred half the screen with plastic wrap to help kep up humidity.  i also use a uth combined with an over-head mercury vapor bulb for hea and uvb.   the mvb is hung above the screen top to help give a larger basking spot. it keps the temps in the right range, and i supplament with nightly misting (i do it for my lizards, so it's not that hrd to do it for him too).  seth brought up some good point, fish tanks make poor snake tanks, but sometimes that's the best we can do for the time being.   i havn't found the right cage for my more arboreal snakes yet.  all the plastic cages i find easily are more for terestrial animals, and that doesn't give me the height i'd like for a price i can afford at the moment.  i would suggest looking at he sites mentioned above and see if you can find what you need.  ultimately, a fish tank doeant give a lrge snake the climbing space it needs either.  good luck with your new guy.  also, my carpets rarely are ever seen by the water bowl unles they are drinking, and i rarely ever even see that.  don't worry about it unles you notice him getting really dehydrated.

Even better Idea .. look at my animals  and click on my cages   ..  take your Tank  flip it on its side  put sliding doors on it  and  add a heat panel to the top  and a UTH  under it  at the same side as the heat panel  ..  Done  and it will look Killer  .. feel free to ask me any questions  I even have the parts  you will need here 

Some good ideas so far, especially Seth's post. I do want to add-- I live in a very low humidity area (high alpine desert, 8000 ft) and I never have a problem with any of my carpets shedding. Internal hydration is much more important and I don't generally mist. In the winter I try to have a humidifier running, but its for the green tree pythons' benefit. The only time I've ever seen my carpets drink is when I put a clean bowl with fresh water in the cage-- so weekly, pretty much, but not every carpet does that. Other than that I don't see them drinking. A more inexpensive option to add is a heat lamp, but radiant heat panels are definitely the best.
MdngtRain, the majority of my cages are plastic for arboreal snakes and they work out great. I have Boaphiles for the GTPs, the arboreal version with a pvc perch from the company and wood perches I installed myself. Two carpets have Constrictors NW cages (however I'm not a big fan of those cages) and one has a shelf (great for large, heavy carpets) and the other had a perch.. but Taliesin kept knocking it down; have to get a better way to mount it. The cages that are 18" and 24" tall are plenty tall enough for large carpets to move around in. I have a cage that 36" tall as well, but its homemade and if it were longer that height would be a bit overkill. Perches and shelves are definitely utilized but overall you do want more of a horizontal gradient.

To increase humidity, try an unglazed flower pot. Dunk it in hot water to raise humidity a lot. Lightly mist with cool water to raise humidity a little.

How to keep temp and humidity stable in a large enclosure? INSULATION. If that isn't an option and you have problems with draft sweeping out the moisture and warmth try getting an exo terra fogger and a timer, for heat you are going to need belly heat as well as basking lamps. Run the lamps during the day and the belly heat at night.