Juvie has sunk-in belly/bowels

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My 7 inch juvie Casper and his tank mate Apollo just got switched into a 55 gallon tank, much better than their 20 g one (yesterday) and they seemed to have been doing fine, but I noticed Casper has a sunk-in belly. I think ive seen him do this before but i cant quite remember when or why. Hes always been picky with food, doesnt like to eat too much so he is a little lean, I know that. I will post a photo in a minute.

Also, yes I tested the water and everything is perfect. The only questionable thing is their switch from bare bottom to sand.

 

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here-

 

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Does anybody know ? ....

 

[Holly12]

I haven't seen this before but maybe someone with more experience would know. I must say though, Casper has adorable little legs!!