-germ-
Rheophilic....
Sorry, agree with RDD to an extent, without knowing the ammonia content its hard to say, if it was me and the fish has been kept (wih its old owner as well) in mineral water then i'd certainly think performing a 50% water change comprising 25% mineral and 25% tap is good idea.
Edited to add-->Then repeat the same process in a few hours (i'd go for 3-4 but its just a guess)
I don't think the OP has dechlor though..... Although you may be able to help with the dechlorinating without dechlor RDD?
Just moving him into new water is too risky IMHO, may lead to stress and/or osmoregulatory issues...
Add gravel vac to your list rainbow if you don't already have one
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RDD you joining the Ninja squad as well?
Edited.... for reasons of my own
Edited to add-->Then repeat the same process in a few hours (i'd go for 3-4 but its just a guess)
I don't think the OP has dechlor though..... Although you may be able to help with the dechlorinating without dechlor RDD?
Just moving him into new water is too risky IMHO, may lead to stress and/or osmoregulatory issues...
Add gravel vac to your list rainbow if you don't already have one
-----------------------------------------
RDD you joining the Ninja squad as well?
Edited.... for reasons of my own