Let’s start by problem solving.
Are you certain it is a Ribbon snake? There is a chance you are feeding the wrong species the wrong diet.
Does the snake have enough security? A nervous snake will often go off feed. Make sure it has an appropriate hide box. Do not handle or disturb it more than necessary until the snake has established itself. Ribbon snakes can be nervous animals.
How small is the feeder fish and how large is the water bowl? Maybe the snake does not detect the fish or is unable to catch it.
Is the snake warm enough? Low temperatures will also throw a snake off feed.
Does he look healthy? Unfortunately it may be that the snake is just sick or terminal. The fact that it has defecated since you have acquired him is a good sign this is not the case.
Get rid of the crickets. Uneaten crickets can harass (cause stress) and even injure a reptile. Whenever crickets are introduced to a reptile’s cage it is a good idea to put in a piece of carrot or something to feed and water the crickets. I have a theory that this practice reduces the frequency that the crickets will try to obtain food or moisture from the reptile’s soft tissues. Crickets are capable of biting at the eyes and mouths of snakes and lizards, and have been known to do so.
Your best bet at this point is probably appropriate sized feeder fish. Make sure there are enough that he can catch a few. You may want to lower the water level in the bowl significantly to make it easier for the snake to catch them. He shouldn't have a problem fishing anyway but you will at least rule out the snake’s failure to catch fish. I have a Ribbon snake that feeds well on scented pinkies (about 1 large pinkie a week) placed at the opening of his hide box. This is much easier, cheaper, and nutritious in the long run. My snake has excellent color and weight, very healthy looking. You may want to try and convert yours to frozen/thawed pinkies but only after you get him to eat regularly. Feeding rodents also cuts down the risk of internal parasites. My Ribbon has yet to ever take an unscented pinkie like many Garter snakes will.
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Randy Hallman