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What do you feed and how much? What is his habitat like? Any other symptoms besides the lethargy, limpness and coolness?
According to http://www.wnyherp.org/care-sheets/snakes/garter-ribbon-snake.php . There are a few adjustments you may need to make in his feeding and habitat that can restore his normal vigor:
Heat/Light: It has not been proven whether these snakes require UV light or not, though they should be provided with an appropriate photoperiod. Basking areas can be achieved by using incandescent basking bulbs, infrared heat bulbs or ceramic emitters. Under no circumstance is a hot rock acceptable since these will often cause serious burns to your animal. Timers and thermostats can be used to control photoperiods and temperatures in the enclosure. The cage temperature should be on the cool end of the enclosure should be around 70° F with a basking spot of around 84° F. At night the basking area should be turned off.
Environment: A suitably large enough pool should be provided for the snake to allow it to swim. A hide box should also be provided. Branches for climbing can also be provided,
Diet: These snakes eat a varied diet. Items such as earthworms, red worms, and other insects will usually be readily accepted. Fish such as guppies, sand eels and Lance fish can also be provided in the pool. An exclusive diet of just fish is not nutritionally complete and should be fed with other prey items such as earthworms. Many specimens can also be converted to appropriate sized mice. You also may want to increase his feedings to 2-3 times a week.
Also, if his symptoms do not improve then you should take him to a vet that specializes in reptiles and exotics ASAP.
Hope this helps!
Goldfish are lacking in nutritional value so your snake may be suffering from a vitamin deficiency. You can try feding slugs as they are high in calcium. Since your snake only eats fish he will need to be fed a varied diet. If you switch him to feeder fish this will help, also try incorporating slugs and crickets. You also may have to add vitamins to their food to make up for any they are lacking in their diet.
Have you tried feeding him mice, they are not "natural" food for ribbon snakes but they are more nutritional than fish. You should not use live mice,only fresh thawed should be used. Pinkie mice work fine. The below site has good tips on training your snake to eat them. Also, the advantage to them is because they are more nutritionally complete your snake will not have to be fed as often.
Also, if his cage is not warm enough this can account for the lethargy so make sure those temperatures are right.
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