You won't be able to keep a manderine in a 15 gal
unless it is one of the rare ones that will eat frozen foods. Mine is slowly starving in my 110 gal
with an ample supply of pods
. Although he does eat frozen, he is a very slow eater and is lucky to get 1 piece of shrimp eaten before everyone else eats it all.
He reminds me of the kid in the (McCain's) frenchfry commercial who is slowly eating each fry individually. Pick 1, set it aside, apply ketchup, put ketchup aside, take 1 bite, chew, take another bite ... and so on. My manderin will stare at the shrimp, take a bite, spit it out, stare at it, take a bite again if it hasn't been stolen by then.
All that to say a manderin in a 15 gal
will likely starve as most won't eat frozen at all.