I believe they start smelling bad when they are dying, that's what I was told when I lost my first one.. I would think fishy smell is not bad..
Nems are so weird sometimes, feels like they go through life changes.... Lol....
I hope yours is ok, also I see some color, that's good too.
I don't know much yet, but I thought I would give my idea... Good luck with him...
A nems mouth should not be open unless it is expelling waste--brown stringy stuff--if its white stringy stuff u see hanging out thats its insides and it is dying.I had 2 green bubble tips that did that--and one even lost all its tentacles--i sadly had to get them outta there because at the time I had no idea why they were dying--so I had no idea how to make them better.We decided to do some research and thought maybe it was the temp--our temp would swing from 79 to 83 everyday.We bought a small fan and put it blowing on the sump--now our temp is 79 at night and 80 during the day.We decided to try one more time with a rose bubbletip(we got it at a great price) and it seems to be doing amazingly!! It eats very eagerly and its mouth is always tightly shut when not eating.Good luck to you and your nem--my advice--keep an eye on temp swings--hope this helps!!