API master kit says Nitrate of tap water is zero, tank Nitrate has come down a little to just above 20.
Does the flow rate impact at all how well the biological filter deals with nitrates? I had set the pump to the highest flow rate (600l/hr) to generate more oxygen. Having looked online at Ex-Nitrate by Seachem, it says the flow rate should be no more than 200l/hr for this product to work. Is this the reasoning behind it?
Just a quick addition to say both ammonia and nitrites are zero with the API master kit... Nitrates are ca 30. It's still puzzling to me why nitrates are so high, and why the young cory have now all died...
Thank you so much for the reply! So, it does sound to you like it's stress related in conjunction with some water parameters? Unfortunately I only got tests once the problem had occurred and used tetra dipsticks for ammonia and for pH etc, then a liquid tetra test for oxygen and a liquid nutrifin test for nitrate....the api master testkit will arrive only tomorrow so potentially the dipsticks didn't give an accurate result for ammonia and nitrites? (both tap water and tank were 0, and tap water nitrites 0, nitrates minimal) My dechlorinator is Tetra too, the blue stuff with anti stress ingredients. Regarding the algae, would you be able to point me to that thread? There's only one type of algae in the tank, a little on the ornaments and on the side, see photo, it's like very thin brown patches that I rub off weekly. Thank you once again for your input!
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.