Usually when geckos have really skinny tails they have some kind of illness or infection, that appetite does not sound normal. Also, Petco is infamous for selling diseased animals. Make sure you have a warm side of 88-90° with UNDER TANK HEAT. If you don't, correct that problem. If eating does not improve take your gecko to a vet and get a fecal and treatment for any parasites/illness she has.
NOTE: If this gecko was purchased within 15 days call petco TODAY, say gecko is SICK and you want it taken to a vet under their 15-day health guarantee. That way, Petco will pick up the bill instead of you.
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Sarah Stettler aka Starling
Sarah@stargecko.com
StarGecko.Com COMING SOON! Star Quality Leopard Geckos
Specializing in Hypotangerine Tremper Albinos