my friend gave me a unused 5 gallon hex tank, can i make a paludarium out of it?

sure, but you couldnt keep anything in it. no terrestrial animal could live in that little space, and i also wouldnt recommend anything for the water
 
you could easily get away with a couple small shrimp and a snail I think... as long as the water level was fairly high
 
Yeah, i think your options for critters would be limited to shrimp/snails for the water and maybe some mantids or something for the land
 
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