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Why Water Changes During Cycling Are Good

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#1 Bignose


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Posted 15 January 2008 - 09:12 AM

For a long time, I've seen the "common knowledge" that while a tank is cycling, you don't want to do any water changes. (Obviously, this is cycling with fish, not fish-less cycling). One of the reasons given is that you'll take away the bacterial colony's food and the colony won't grow. Or that the colony won't grow large enough to handle the fish's bio-load if you do water changes during the process. Another reason given is that by doing water changes, you'll lengthen the cycling process, and that since the fish in the tank are being exposed to ammonia anyway, getting it done as soon as possible is desirable.

There are many, many websites out there that repeat this same information, Google brings up many examples.

It's time to show everyone that this information, this "common knowledge" is just plain wrong.

First, let me explain the theory behind cycling. The steady state of the situation is well known to us. The size of the bacterial colony is large enough that it consumes all the ammonia the fish produce immediately. Worded another way, the rate of consumption (by the bacteria) is equal to the rate of production (by the fish). Or, to write it mathematically,

r_p = r_c

r_p stands for rate of production
r_c stands for rate of consumption

Let me re-write this equation as (the reason why will become apparent soon)

0 = r_p - r_c

However, while the tank is cycling, the rate of production and the rate of consumption are not equal. Specifically, the rate of production is greater than the rate of consumption (mathematically r_p > r_c). The bacterial colony hasn't grown large enough to consume a large amount of ammonia yet. That leads to an accumulation.

a = r_p - r_c

where a stands for the accumulation of ammonia. (for the real math nerds out there a = dc/dt where c is the concentration of ammonia and t is time)

The real power of this equation and the above one is that it is not the amount of ammonia that is in the tank that is important. It is rates of production and consumption that are important. When the rates are equal, no ammonia will accumulate in the tank, and that is what we read when we take the sample. A reading of zero indicates that the rate of consumption by the current bacterial colony is equal to or greater than the rate of production by the fish.

However, a reading of some ammonia, tells you virtually nothing about the rates in the tank. The ammonia reading is like a snapshot of a moving target. You get a picture of the tank at that exact moment, but it doesn't tell you anything about the dynamics of the situation at that time.

Also, it is very important to note that the rates themselves are independent of the ammonia reading at any time. This can be taken advantage of, because, if you do waterchanges while the cycling process is going on, you can dilute the poison the fish are living in, and not disturb the cycling process. The bacteria can only eat so much in a given day, and until the very end of the cycling process, the bacteria aren't going to consume as much as the fish produce in a single day. For example, if on a certain day, the bacteria can consume 0.1 ppm of ammonia, they don't care if there is 1 ppm of ammonia or 5 ppm of ammonia in the tank. Just so long as there is at least 0.1 ppm of ammonia in the tank, the colony gets as much food as it can consume, it will grow and consume a certain amount the next day. The bacteria only stop growing if there is insufficient food. So, since the bacteria don't care if there is 0.1 ppm, 1 ppm, or 5 ppm, there is no reason not to do a water change to dilute that down -- just so long as you don't dilute it down below 0.1 ppm. Because, the fish very much care if they are living in 0.1 ppm, 1 ppm, or 5 ppm. That this can be done is what I am going to show below.

I set up an excel sheet to simulate this equation. One where I simulate straight cycling without water changes, and one where I simulate doing a 10% water change at the end of every day. I picked r_p = 1 ppm of ammonia per day, and r_c = 0.000014 ppm of ammonia per day for the first day. Then, everyday after after that r_c doubles (approximating that the filter bacteria double in number roughly once every 24 hours). That is, on day 2, r_c = 0.000028, on day 3 r_c = 0.000056 and so on. This choice of r_c on the first day will become apparent below.

day....ammonia reading at end day (ppm)

The tank took 21 days to cycle, and the ammonia got up to 15.16 ppm as a maximum concentration. This is pretty high, and might have needed emergency action at that level. (If I may not too immodest, see my other article to compute when emergency action is needed: http://www.fishforum...Ammonia-Charts/ ) Most test kits don't go up to that high anyway.

Now, let's look at a tank that gets a 10% water change at the end of each day:


This tank cycled one day faster. And the maximum level of ammonia that built up, 7.52 ppm, was less than half of the maximum of the non-water changed maximum of 15.16 ppm. 7.52 is still pretty high, but probably not nearly as bad a 15 ppm. The filter bacteria do not get starved, there is plenty of ammonia to eat every day until the last one when the tank is cycled. In fact, at that point, the bacteria colony has grown large enough to consume 3.67 ppm of ammonia per day, more than 3 times as much as needed -- recall that the fish are assumed to excrete 1 ppm of ammonia per day. And, of course, the objection that cycling takes longer is smashed, because this tank actually cycled one day sooner.

Normally, this is where my argument would end. Personally, I think the math is convincing enough, but I also know the math really well. I know that most others don't know the math as well as I do, or quite get the math. However, Over the last month, I've been performing an experiment. I took my quarantine tanks, two bare 10 gallon tanks, and two extra filters I have and set the tanks up. I performed the experiment that the math above describes.

Here's the details. I did fishless cycles, because then I could control exactly how much ammonia went into each tank. Both tanks were set up the same, and I took the readings at the same time each night. I added the ammonia at the same time each day. I added enough ammonia to be equal to 1 ppm per day. The test kit I used was brand new, and measured from 0 to 7 ppm of ammonia. The gradations were 0, 0.25, 0.5, 1, 2, 4, and 7+ ppm. I had to guess some of the intermediate values, so, I only report whole numbers (no decimals) because they are just guesses. Anything at 7 or above is just recorded as 7, because that is the limit of the tank.

Here's my computational results side-by-side with the experimental results.

Now you see why I used r_c = 0.000014 as the first day's consumption value. It is the value that best fit my data. It is actually a pretty good fit between the experiment and data. The tank cycled on the same day, and the decrease at the end is missed just a little. I got a value of 4 ppm for the 20th day's experimental value, when the computer predicts 5.3 ppm. But, I think that is pretty good. Most likely, the simple doubling rule isn't quite right when the food is starting to become scarcer. Nevertheless, it shows that model used is pretty darn good, really.

Here's the tank with the 10% water changes done every day (I did the water change everyday immediately after doing taking the ammonia readings).


Again, very good agreement between simulation and experiment. The tank did indeed cycle one day earlier than the non-waterchanged tank. The readings taken during the beginning and the simulation agree very well, though again at the end of the cycling the experiment and the computer had some small disagreement. The disagreement is not large, and I think that the general trend is captured really well.

Since I feel the model is pretty well verified now, I ran a few more simulations. A tank that undergoes 25% waterchanges everyday reaches a maximum of 3.82 ppm of ammonia, and is cycled by the end of day 19. A tank that undergoes a 50% waterchange every day reaches a maximum of 1.99 ppm of ammonia and is cycled by the end of day 18.

I am fairly confident of the 25% water changed tank, but not sure of the 50% tank. The levels are kept very low in that tank, and since the bacteria don't grow exponentially (doubling every day) when the food levels are lower, it may not be exactly right Nevertheless, the idea is very similar, you can keep the ammonia levels low without sacrificing speed of the cycling process. Keeping the ammonia levels low keeps the fish in much better health than otherwise.

For example, see "Low levels of environmental ammonia increase susceptibility to disease in Chinook salmon smolts " by Ackerman PA, Wicks BJ, Iwama GK, Randall DJ in PHYSIOLOGICAL AND BIOCHEMICAL ZOOLOGY volume 79 JUL-AUG 2006) which showed that fish exposed to low levels of ammonia were more susceptible to disease later on in life. Basically, exposure to any levels of ammonia leads to greater health problems for the rest of the fish's lives. But, the lower that exposure, the less susceptible the fish will be later in its life.


In conclusion, I have given both experimental and computational evidence why it is not bad to perform waterchanges while cycling, and indeed actually beneficial. The experimental evidence I think is particularly compelling, it is no longer just a theory or a mathematical construct that I came up with. The theory matches the experiment pretty well. The experimentally waterchanged tank cycled one day earlier than the non-waterchanged one. Unfortunately the test kit couldn't measure high enough to measure the peak concentration each tank go up to, but I think that the very good matching of the curve up and then the pretty good matching of the curve down is excellent.

So, everyone, please do waterchanges even when cycling. Especially at the beginning when the bacterial colony is just starting to grow and can only consume a very tiny percentage of the fish waste every day. At that stage you can do very large water changes to keep the ammonia level down then and later. It doesn't matter because the bacterial colony will grow until the rate of consumption is equal to the rate of production. And, so long as the temperature, pH, and hardness of the replacement water is the same as the tank water, there is no harm in doing large water changes, 50%, 70% even 90% if you have the capability.

#2 kewskills


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Posted 15 January 2008 - 09:22 AM

Wow, thats quite a read, but very interesting, thanks for posting.

#3 andywg


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Posted 15 January 2008 - 09:56 PM

Excellent. I like this addition of personal experiments to back up the theory. Nice work.

#4 rdd1952


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Posted 15 January 2008 - 10:35 PM

Great work and post. :cool:

The bacteria development should actually be the same whether you do water changes or not (they will consume as much as they can and double every 24 hours or less) but the time required will be less (though only slightly) simply because there is less ammonia in the tank when it peaks/spikes meaning less bacteria are required to return the amount to zero. The fact that the time required is less really isn't the key issue, it's the fact that the level of toxins that the fish are exposed to are much lower. So it has always made perfect sense to me that water changes would definitely not slow the process but as you said, that myth is everywhere.

#5 Bignose


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Posted 15 January 2008 - 10:46 PM

Yeah, rdd. I wanted to address every aspect of the myth. The most important thing to me, too, was that the toxic levels were kept much lower, but many people worry about the total time of cycling, so put that in there too.

If there are anymore aspects of this myth that you (or anyone else for that matter!) knows about, that I didn't address here, please let me know about them. I want this myth to go away so that everyone feels comfortable doing large water changes while cycling to help prevent the fish's exposure to ammonia and nitrite.

#6 businesslamb


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Posted 15 January 2008 - 11:09 PM

Nice data bignose. Always good to see some real evidence to prove or in this case disprove "percieved wisdom".
I take it you're a scientist of some description?



#7 Snooks1!


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Posted 15 January 2008 - 11:44 PM

Kick-jerk read mate! Really top notch work, very informative and provocative results. This is really sticky-worthy. :good:

#8 celaeno



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Posted 16 January 2008 - 12:39 AM

bignose, doesn't your computer and experimental data also suggest that cycling with fish and fishless cycling both take just as long?

that would mean the end of another myth, namely that fishless cycling is quicker than cycling with fish.

#9 Bignose


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Posted 16 January 2008 - 01:29 AM

bignose, doesn't your computer and experimental data also suggest that cycling with fish and fishless cycling both take just as long?

that would mean the end of another myth, namely that fishless cycling is quicker than cycling with fish.

Yeah. I've personally never found one method to be faster, but then again I only cycled with fish the very first time when I didn't know any better. But, it's always been spot on 3 weeks for me. Every tank I've fishless cycled since then has been within a day or two or 21 days.

There is one aspect of fishless cycling that is potentially faster. That is, because there aren't fish in the tank, you can turn the temperature up to the mid 80s (Fahrenheit), and allow the bacteria to reproduce at a slightly faster rate. At normal mid 70's temps, the cycling bacteria double at around 20 hours, and I think in the mid 80's it increases to around 17 or 18 hours. Let's call it 2 1/2 hours. If cycling takes 21 days, saving 2 1/2 hours per day adds up to just under two total days.

I wouldn't consider two days to be hugely faster -- and two days are well within the experimental error of telling someone "3 to 4 weeks". I think that with the increased temperature, then fishless is a little faster.

But, comparing the two, I don't think speed is the primary concern -- it's keeping the fish from being exposed to toxins. And, while I've shown how doing water changes keep the pollution levels pretty low, they definitely aren't zero. And again, I can't stress this enough, that exposure to ammonia -- at any level -- weakens a fish's immune system permanently at least a little bit. So, if there is ever a choice, fishless is far, far superior in my mind.

#10 jourdy288


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Posted 16 January 2008 - 01:39 AM

Wow! I wish I knew that earlier.

#11 rdd1952


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Posted 16 January 2008 - 02:43 AM

Just an addition to what Bignose said about cycling with fish vs fishless cycling, there is also a lot less work involved in fishless cycling, namely no daily water changes. But as mentioned, the main purpose is to protect the fish.

Edit: I decided to read the "Similar Topics" at the bottom of this thread. I think it's funny that they all seem to ask the question and the opinions differ. This should lay it to rest for good.

Edited by rdd1952, 16 January 2008 - 02:49 AM.

#12 pastabake


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Posted 06 March 2008 - 10:51 AM

Brilliant bit of work Bignose.

I've just started on a fishless cycle using the method from the link in rdd1952's sig. Am I right in assuming that I'd do just as well by adding 1ppm daily and at day 19 or there about start testing for nitrite and ammonia thus saving myself the time and expense of lot of intermediary testing?

Also how do you factor in the ammonia uptake from plants ... should I just add an extra 1ppm daily?

Edited by pastabake, 06 March 2008 - 10:52 AM.

#13 Eclecticoldsod


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Posted 06 March 2008 - 01:03 PM

Great post – not that I understood the math (but my mental curtain came down on that subject at long division..!)
When me and God were children, I was told to do it that way. No one knew why, it was just the way it had always been done. I don't think any one knew much about water chemistry in those days – a few hundred years of trial and error had worked out the best way to do stuff.
“Set up the new tank using plants and gravel from an old tank – wait two weeks – add a fish – wait three weeks – then you can gradually add more fish (no more than one a week, mind...)”
It was hard for an impatient child to wait so long, but we was obedient then - “You either do it like uncle Fred says, or you won't bloody-well have a fish tank!”
It worked for me, so I've always done it that way.

Nice to find that science confirms what a ten-year-old boy always thought was an old wife's tale designed to teach patience/punish small children... :good:

#14 Bignose


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Posted 06 March 2008 - 11:36 PM

Brilliant bit of work Bignose.

I've just started on a fishless cycle using the method from the link in rdd1952's sig. Am I right in assuming that I'd do just as well by adding 1ppm daily and at day 19 or there about start testing for nitrite and ammonia thus saving myself the time and expense of lot of intermediary testing?

Also how do you factor in the ammonia uptake from plants ... should I just add an extra 1ppm daily?

You definitely don't have to test every day. But, most people like to feel like they are doing at least a little something with the tank.

I am sure that the uptake from plants is going to be quite variable. Depends on the plants species, number, how healthy they are, temperature, how much light they are getting. This is probably why you should test ;)

Edited by Bignose, 06 March 2008 - 11:36 PM.

#15 cheesy feet

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Posted 06 March 2008 - 11:50 PM

Definitely pin worthy, well done :good:

#16 n3ont3tra


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Posted 07 March 2008 - 03:24 AM

Great post. Very interesting. I'll even put it on my siggy.

#17 cc_woman


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Posted 22 March 2008 - 03:02 AM

Thank you for all the useful information. It will help me out for future times of buying any new tanks. I just love hearing smart people talk, people who actually know exactly what they are talking about. It bothers me when I get bad information on something, or I see others giving bad advice. I think this forum is great for that.

#18 Piltdownpaul


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Posted 22 March 2008 - 07:03 AM

I've never had a problem doing large water changes in the cycle-In fact i didnt realize it could be a problem thinking the bacteria were multiplying on substrate etc. My 88 litre tanks cycled in 4 to 5 weeks so it cant be lengthening the cycle. Good post knowledgeable friend!! :good:

#19 pastabake


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Posted 09 April 2008 - 01:09 PM

Nice one Bignose. I followed your idea and limited my ammonia to +1ppm per day with a max concentration of 5ppm at its peak and my tank cycled in 20 days. I was going to ramp up my ammonia at the end by adding 2,3,4,5ppm post cycle so as to be able to add a full stock of fish but an unexpected hospital visit put an end to that and by the time I got back the tank was being used for fry. Still its good to provide further evidence.

#20 newfishy


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Posted 09 April 2008 - 01:28 PM

wow, excellent work there.

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