Hi,
Reptarium cages are inexpensive:
http://lllreptile.com/store/catalog...nd-explorariums/-/100-gallon-basic-reptarium/
Converting a fish tank to something appropriate is going to cost more than getting a new enclosure.
I think one of the main issue is feeding the crickets. They need food, not just "quencher". Also, it doesn't sound like you are giving him enough crickets, though it's a little hard to tell from what you wrote. The food that's in the cricket's stomach is what your chameleon is eating.
When you get him in a taller enclosure, the top should be around 90 degrees, the bottom in the 70-75 range. You want him to have a chance to move in and out of warmth.
At night temperatures should drop to the 60s.
My suggestion until you can get a new enclosure: ditch the heat bulb entirely. Get a low wattage incandescent bulb (the kind you can buy at the grocery store, with your current set up I'd say buy a 40W bulb and a a 25W bulb and try the 25W first, if you get a temperature above 85, stick with the 25W). Put the incandescent all the way to one side of the enclosure, don't center it. This will hopefully promote airflow.
They can be remarkably resilient so please do not give up on your pet.