My vivarium is 24" wide by 12" deep and high. I am not new to reptiles and amphibians as i care for two white's tree frogs and have cared for leopard geckos
Have you considered a Viper gecko? They are small, docile, easy to care for, interesting looking little animals that would do quite well in an enclosure that size.
Here is a care sheet for further information on them. 
http://www.vmsherp.com/CarePages/CareTeratolepis.htm
I hope this helps, and if you have any further questions at all on the subject; please feel free to message me and I will be happy to assist you.
Good luck!
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About Tricia
you can have a leopard gecko, they are ideal or even a water dragon.
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Have you considered a Viper gecko? They are small, docile, easy to care for, interesting looking little animals that would do quite well in an enclosure that size.
Here is a care sheet for further information on them.
http://www.vmsherp.com/CarePages/CareTeratolepis.htm
I hope this helps, and if you have any further questions at all on the subject; please feel free to message me and I will be happy to assist you.
Good luck!
References :
I've kept reptiles for almost thirteen years.
do not put a water dragon in there it is way too small!!!!!!!!!!
something small along the lines of what you have had will fit but it is quite a small enclosure
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i agree with the above answer, a baby water dragon will fit but not for long, spiders, scorpions, and small breed geckos would be best. Have you considered a crested gecko? they are awesome little guys.
Best thing to do would be bring the tank to your local reptile shop and ask them to point out the ones that would be good for you.
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anoles and maybe some gekos
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I'd like to clarify that a viv that size is really only big enough for ONE of any small reptile you decide to put in there.
An anole would be a good idea, but they need height and climbing area. If it is a tank, you can turn it on its side to make it tall rather than long.
A toad would be a good idea as well.
One of a number of small geckos could do well in there.
Please remember to do your research and only get the a reptile that doesn't have complicated heating needs, as it's often difficult to get a proper heat gradient in a tank that small.
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Many years of herp experience.
There are a lot of things that have to be accounted for when you have a reptile as a pet. One of my good buddies growing up had a python. He had set up a mini resort for the snake. It was pretty awesome how much detail he put into it.