Hi. Have an issue with nitrate levels causing death to some fish. Tested water with Tetra test 6 in 1 test kit after three mollies died in space of ten minutes. No3 and No2 nitrate and nitrogen dioxide levels are off the scale. Have always had cloudy water as well. So carried out two 30 percent water changes in space of two hours. Cleaning algae from side of tank and a good clean of gravel. Water going into tank has perfect readings. Pre added to new water ph balance, dechlorinator, healthy bacteria liquid. And correct temp. Still nitrate very high. Next day carried out 50% water change. Again nitrate levels perfect in new water. Removed coral and plastic plants for cleaning. Again cleaned gravel. Changed in external filter carbon for Fluval clear max and also put in Evolution Aqua pure water balls. Now after 12 hours have perfect clear water like never before. No more dead fish of which we have 55 in a 125litre tank of around 10 different species. All fish are very active, eating well but still no2 and no3 levels are very high. I know we may have too many fish but am at a complete loss now as to what could be causing this. Any help will be really appreciated. Thanks

It's not the nitrate that's killing them it's the nitrite. Nitrite at any level is dangerous as it prevents the fish from taking up oxygen by altering the composition of the red blood cells. You need to change as much water as necessary to reduce the level to 0.25 ppm of less every day until this resolves and treat the replacement water with a double dose of Seachem Prime for the whole tank volume to protect the fish over the 24 hours between water changes.

You are very seriously overstocked and this is the cause of the high nitrite,nitrate, cloudy water and all the other problems. You are likely to have ammonia present as well, but you test strips don't test for that. AN ammonia test is vital, but don't get paper strips as they're inaccurate.Get an API liquid base master test kit which will give you more accurate results for nitrite nitrate and pH as well as ammonia.

Long term you need to rehome a lot of your stock or this problem will keep recurring.

Thanks for the advice. I have the test kit you said and the prime seachem. Have given the whole tank a double dose. Have carried out 3 35 litre changes in the last 24 hours. Always testing the water before it goes into the tank. Just finished the third water change and tested the nitrite straight away. Still off the scale. Could this be something in the ffilter?

No - just way more fish than the filter can cope with.

Ok. Let me give you some background. Roughly three weeks ago I fitted an external Fluval 206 filter. Could it be the filter is not fully cycled yet ?
The reason I say this is because ammonia, nitrate, ph are all ok.
I know I have too many fish. I have the old filter which is a Fluval u3. Would it make sense to have this internal filter running also ?

Did you move the media from the old filter into the new one? If not, the new filter would take weeks to cycle and the fish would have first been exposed to ammonia and then to nitrite. This would explain why you still have a nitrite reading. Until the nitrite is back at 0 and staying there follow this guide: http://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm

High nitrAte is down to too much stock so to deal with that you will need to do larger and more frequent water changes indefinitely to keep it under control.

Adding in the old filter now won't help as the beneficial bacteria it contained will all be dead so it is also not cycled.

Thanks again for your advice really appreciate it. We have had really bad advice from various places and the poor fish have suffered as a result. Thought would give an update as to what is happening. Have been testing ph, ammonia, nitrite and nitrate every morning. Generally what I'm seeing is ph ok, ammonia sometimes rises a little to 0.5ppm so when I'm seeing this treat it with api Ammo lock. Nitrite still off the scale 5.0ppm and over. Nitrate is ok at around 10ppm. So I'm carrying out 35% water changes sometimes twice a day am and pm. The fresh clean water going into the tank gets 8ml Fluval water conditioner, 5ml of seachem prime, ph powder and 5ml of Fluval bio enhancer. Once the water change has been done, I'm adding another 5ml of prime seachem to the main tank to try and protect the fish a little from the high nitrite.
So I'm thinking I will continue on this course until nitrite eating bacteria grow and start to see nitrite levels reduce. Am keeping feeding to a minimum like every other day and keep lighting on for six hours only.