First, good for going to the vet, but I am confused about the behavior of the vet and I would suggest you attempt a second opinion.
A fecal is great, but it doesn't necessarily catch everything and you have to get a really fresh one. What I am confused about is if you took the gecko to the vet why they didn't take a swab... the technical term escapes me (having a bad day myself) but basically they stick a tiny swab up their back end and test the swab for parasites. This was how we found out our departed little one had parasites. It isn't as intrusive as it sounds, BTW.
Also if the gecko isn't eating well, the vet should have given/sold you hand feeding formula... basically it is baby formula, ground dried crickets, and vitamins, it is used for force feeding very sick geckos.
I am very concerned about the behavior of your little one, it is very sick. I would follow through with this vet for sure, but I'm really disappointed in the vet's procedures and would highly recommend trying another vet. We had to try a few too until we found one that really knew her stuff. Good luck, I'm hoping for a speedy recovery.
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0.1 Albino Leo Gecko
0.1 Crested Gecko
1.0 Dear Boyfriend
Departed: Harvey and Spock