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FISH DYING WITH NORMAL TANK READINGS?

Subject: FISH DYING WITH NORMAL TANK READINGS?
by emmawhitmell on 2018/10/20 16:55:32

100L tank

ammonia: 0ppm
nitrites: 0ppm
nitrates: 5ppm
PH: 7.4 - 7.6
temp 24 degrees

also took water to fish shop who tested it and said the same, no ammonia, no chlorine

tank has been established for months, live plants, no issues whatsoever

monday 15th - came home from work, 2 out of 7 of our tiger barb were dead, 2 out of 6 glowlight tetra dead, panda & peppered cory and red tailed shark absolutely fine

tested water, everything normal, nitrates slightly high, maybe 15ppm so I did a 30% water change, retested and they were down to 5ppm

fish continued to die over the next few days, gasping at the surface for air, gills red in some of them, but still the tank tested fine, some looked like they had a swim bladder problem

our last remaining tiger barb started growing a fungus on it, i treated the tank with lovefish bacteria & fungus treatment on thursday, fish died on friday

as the tank tested normally and everything looked fine we bought some neon tetra on friday hoping that if it was disease that killed the fish it was eradicated by thursday's treatment - they were all dead this morning

currently have 1 remaining panda cory in the tank, and a peppered cory which looks like it's dying - very lethargic, some yellow rabbit snails and some amano shrimp

question now is what do we do? obviously won't introduce any more fish but we have no idea what the problem is so don't know how to fix it. Any ideas greatly appreciated

Thank you