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Cory heavy breathing and dying

Subject: Cory heavy breathing and dying
by Pipmeister on 18/8/2020 16:21:16

Hello all,

Thank you for reading....I wonder if anyone has any ideas what is wrong with my fish tank. The tank was set up 3 months ago with our two old bronze cory, aged 4 and 6 years. It was started with Fluval Cycle (daily, then weekly) and weekly 20% water changes, plants and ornaments (soaked for 1 week before adding).

Last week I added 5 young cory and all of them died, some were breathing heavily and stayed at the bottom, others behaved normally but suddenly started floating belly up and appeared weak. Within 3 days all perished. The 2 old corys appear to be just fine thankfully.

Since then I've done another 30% water change, increased oxygenation with an air pump and checked water parameters, here's all tank info:

Fluval Flex 57 with manufacturer's filter incl. Carbon pad,
Temp: 24.5 C,
4W air pump /air stone
Oxygen: 2-5
Ammonia+Nitrites: 0
Nitrates: 20-50 (fluctuating)
Ph: 7
Chlorine: 0

I used to add Seacham flourish excel 1ml every other day but stopped since the young corys became unwell.

After another 20% water change to lower nitrates I've added 3 young cory and one of them has started breathing heavily on day two, another one has died overnight . I'm at a loss how to remedy the low oxygen and high-ish nitrates. Your advice is much appreciated!

Photo of the tank attached, all fish hiding under the ornament.

Thank you!