Sorry for your loss, something is definately not right, how long have you had that one, I would get the lfs to take a scrape as that s the second one to die off the same thing.
Put him in yesturday mate.
Its identical to the old betta.
But all other fish are absolutley fine!!!
Its very odd.
My water is at its best.
0 ammonia
0 nitrita
0 nitrate
The shop tested it when my other cardinals died and said its fine too.
If it was that diesease your on about, I would have thought my other fish would have died by now if its that fast acting.
No I lost all my guppys to columnaris when first started, as my husband bought one with it, wiped them all out but other fish in the tank like tetra's were fine, never got it, It could be a parasite, do you have any pictures.
Best thing is to take it back and tell them whats been happening, I'm still learning myself, you can't always tell on a picture, I think the fish needs some scrapes to no what's going on.
how long has the tank been going rodders? it looks like it hasnt cycled yet as you have no nitrate readings. test your tap water from a glass and post those results.
I've got no nitrates cause i got an Interpet nitrate absorbing sachet. I use this becase my tap water has extremely high levels of nitrates in it. Its just good enough to suck it all up
Tank is cycled.
usually bacterial infections cause fin and body rot so maybe you should take that route. just because some of the fish havent died doesnt mean theres nothing there. cardinals and bettas are sensitive to water borne bacteria and maybe using something for bacteria might help before adding any other fish. what is the temperature and ph? for bettas they like it around 80f.
just a suggestion though.