A few questions about the nitrogen cycle


Please forgive the basic nature of these questions!

So, I am learning more about the nitrogen cycle. It seems pretty straight forward: food and waste become ammonia, which becomes nitrites, which eventually become nitrates--all three of which are toxic to fish. Here's where I am confused: is the goal eventually to have a tank with 0 ammonia, 0 nitrites, and trace amounts of nitrates (indicating that the cycling process is complete)? Secondly, why, during the cycling process, do fish die so readily without water changes even if ammonia, nitrites, and nitrates are at zero? I lost over 20 fish while going through a fish-in cycle with weekly water changes (which I now know wasn't enough--thank-you so much to everyone who replied to my previous thread!), even though my ammonia never exceeded 0.25 ppm and my nitrite/nitrates remained at 0. What exactly was killing them? Lastly, when the cycling process is complete, and nitrate begins to increase, is the only way to counteract that large water changes? Or do we start using other additives at that point?

Thanks again for all of your help. I haven't lost any more fish since I started daily water changes, and the ones I have left are looking a lot happier!


You are correct in that all three are harmful to fish, but nitrates are less harmful than ammonia and nitrites and therefore can be tolerated by fish. There is some debate as to how high the levels can be but personally I like to keep nitrates below 20ppm - that’s just me though. Once fully cycled, you can keep nitrates in check with regular water changes, avoiding overstocking your tank and adding live plants to consume the nitrates. There are other products available for that but the three things mentioned should be sufficient.

In terms of what what was killing your fish, without knowing more details it will be difficult to provide an answer.


You may not know it but I am very proud of you. I know you got off to a rocky start. What we don't know, we don't know. I am very glad you came to us and now you are well on your way to hopefully understand why the original fish didn't make it. We will probably never know exactly why you were losing fish to begin with though. I do wish we could pinpoint a specific reason but unfortunately we may never know. I have to wonder if it was from lack of proper acclimation. We have to slowly acclimate new fish to the parameters of the water in our tanks.

I don't remember if I asked in the previous thread what you were using to test your water. It seems strange to me that while doing a fish in cycle and only doing small water changes once a week that the ammonia never went above .25ppm. Normally a build up of ammonia while doing a fish in cycle is at the bottom of fish dying during a fish in cycle.

As carsonsgjs pointed out both ammonia and nitrites are toxic to fish. Nitrates not so much. It isn't unusual for mine to go to 40 or sometimes even higher between my 50% weekly water changes. Most of my tanks are heavily stocked so my nitrates are going to get pretty high between water changes.

Lastly, when the cycling process is complete, and nitrate begins to increase, is the only way to counteract that large water changes? Or do we start using other additives at that point?
Water changes alone are the most efficient way of lowering the nitrates. There are products on the market that claim to help. I personally know of one that actually works but I don't recommend running it constantly. Doing so not only pulls out nitrates but could also be pulling out the good things in the water. I don't recommend running any kind of chemical filtration for any length of time.

There is more to doing water changes than just lowering the nitrates. There are things we don't normally test for that can gradually build up in our tanks. Weekly water changes prevent that. Water changes also replenish the minerals normally in our tap water that are necessary for both us and our fish.

You can of course run the nitrate test and let the number tell you how much water to change or you can just go ahead and change out half the water each week and then once a month change out even more. As long as the water conditioner is added and the water is temp matched we can never give our water pets too much fresh water.

I owe the fact that I've never had to deal with any of the fish deseases I read about here on the forum to the amount of water I change each week. Don't get me wrong, like everyone else I do lose fish from time to time. That is the biggest downside to this hobby.


Why fish die is a mystery that without autopsies is really just educated guessing. Remember most tropicals are imported and may go through an incredible amount of stress before they reach the store. Most wild animals carry parasites that they can live with but kill them when their stressed. Even in established tanks most deaths are the newly added fish. We used to think ph shock was a big factor if stores water was diff from yours. Many think this isn't a big factor nowadays. Another possibility is rapidly spiking ammo/nitrite levels that you may have missed. My guess is many deaths may be caused by ineffective/out of date Chlorine remover. Another though is who knows whats really in out water supply. Are their chemicals in our supplied water harmless to humans but harmful to fish that dissolves like chlorine does after a few days? This is a long way to say i don't know. There are many nitrate removers on the market, i haven't used them so not sure how effective they are. Nitrates aren't deadly to fish until they get way up there. I've seen them over 100 and the fish looked ok.

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