I have a fish who died mysteriously yesterday, was wondering if anyone had any ideas on what killed her, and what action i should take to ensure the rest of my fish will not meet the same fate.
its a 30 gallon freshwater tank, with a hob filter (sized for a 50 gallon tank), its been running a little over two years. the nitrates, ammonia, and nitrites are all within normal parameters. i never miss a water change.
the tank is home to one red tailed shark 5 leopard/zebra danios, and 1 roseline danio.
the fish that died was my second roseline danio. i noticed yesterday while doing a water change that she was breathing rapidly, sticking its mouth out a lot. she wasn't swimming very fast- which was unusual, because she typically swims very fast with her buddies. upon further inspection i saw her right side of her body had an odd shaped protrusion. it looked like something was swollen or bloated...or like she had swallowed something large? she was dead within another hour, floating on the surface.
all the other fish seem fine. they have been living together hapilly for a year. so what happened? should i do anything?