Need help in a situation I'm a bit puzzled with. I have an established, 3 foot (90 cm) bare aquarium with discus.

I have added a few plants in the last 3 months. Once the new plants have adapted to the water parameters of my tank, I have perfect water parameters.

This is done by checking it with API freshwater test kit. I have been doing regular water changes of 50% every week... Be it 20% every 2 day, or 30% every 3 days, or 50% during the weekend, all depending on my free time... but this is without failure, it's been my routine.

After the 2nd month of having the plants in, I've noticed ammonia, nitrite and nitrates reading are all 0 before my water changes. With this info, I have dragged my water changes just to see how long I can get away without any water changes.

To my surprise, I managed almost 3 weeks without doing so. Until, I decided to do a 50% water change (with the readings of all zero), 3 days ago. That's when my problem started.

The first night, all was good. On the 2nd night, my fish started to stay at the bottom of the tank, almost all of it and by the morning of 3rd day, my discus have mostly turn black. Knowing something was not right and getting worse, I did a check and got a reading of between 20 to 40ppm of nitrates reading roughly around 24 hours of new water change... (colour seems to be in between the API chart.)

I did a water change again today, put some salt in... And now it's a waiting game.

Any explanation on how did I manage to get this nitrate spike?

Thanking you in advance.

  • 1
    did you use water conditioner when you changed water? i find it strange that you have a NITRATE spike,we do water changes to bring nitrate levels down.you do not need to add salt if the NITRATE is elevated,salt is used to protect against NITRITE.how high is the nitrate level is it over 40-50 ppm? is the levels of ammonia and nitrite elevated too? Jul 10, 2020 at 16:12
  • 1
    where do you get the water from well-lake-stream? any heavy rainfall during the last month or so,please edit this information into your question. Jul 11, 2020 at 7:36
  • As others have asked, can you in detail explain how you create your water change water, if it's filtered, treated, sits/seasoned, etc...? Also, for expensive fish, it's worth it to get high quality testing kits, salifert is my favorite but there's other very good ones. API is often not much better than a coin toss. Also, what's your reasoning for putting salt in? Off the top of my head, you likely have chloramines in your water supply that you aren't properly treating and seasoning. The ammonia in chloramines will cause an increase in NO3 if it isn't neutralized.
    – Jestep
    Jul 11, 2020 at 19:20
  • Thank you all for the reply. Ammonia and nitrite are zero. Water used is treated with anti chlorine. I usually will put two tablespoon of salt. Water used is from my tap supply,as usual, comes in to my 150 gallon tank on the roof,most likely,its a maximum of 1 day old since the same tank is use to run the tap in the kitchen and toilet. Yes,it is a rainy season now in kuala Lumpur, Malaysia.its been so for the last month. I have since thought about it also that it may be chloromine issue,maybe all of my good bacteria have died off, probably. Problem still persist. Jul 12, 2020 at 0:21
  • 3 of my young discus,1-2 inches in size have since died. Fishes seems to be covered with a slimy white coating. I have since ,done another 50% water change yesterday, without salt cause the readings are still the same, nitrate 20-40ppm. Thinking that it might be an issue with chlorine,and maybe i have killed off al my good bacteria in the filter,i have since taken used filter media and sponges from my arowana sump to be place in the filter. Jul 12, 2020 at 0:22


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