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JohnsCreekGA
02-12-2014, 01:24 PM
Water Changes (Numerical Analysis of Frequency & Amount)
All, I have often wondered about the wisdom of 20% weekly water changes to reduce nitrates and fish secretions that stunt their growth if not removed. It seemed insufficient in my mind. As a Finance guy, I decided to do the analysis assuming a base case of 100 nitrate inputs/waste added per week over a sample 20 week period the results are the same if you go longer. My analysis shows that frequent large water changes are the best (obvious), but infrequent large water changes are far superior to weekly small water changes. Note, I will make the comments below under the assumption that you are keeping fish that are not sensitive to large water changes. I can share data with anyone who wants me to email it to them the site would not let me post the charts as it said it was too much data.
First, the analysis shows that with small frequent water changes, the waste will continue to build until it reaches a stable but high level this is true no matter how small or infrequent the changes, but the stable level will be high too high in my mind. A 20% weekly change will result in a range of 4 to 5 times the weekly inputs. A 40% weekly change will result in a range of 1.5 to 2.5 times weekly inputs twice as good right before a water change and almost three times as good just after a water change. An 80% weekly water change will result in a range of only 0.25 to 1.25 times weekly inputs. This is 4 times the amount removed as a weekly water change but the results are significantly more than 4 times as good for all but the time just before the water change. Fish health is not measured by the quality of water right after the water change, but by the average during the week.
Next, I took a look at what happens when you do changes every other week. 20% water changes results in a range of 7 to 9 times weekly inputs way too high in my opinion! 40% water changes every other week result in a range of 3 to 5 times weekly inputs even though this is the same amount of water removed as 20% weekly water changes, it is superior to 20% weekly water changes for the majority of the two week period and never gets worse. 80% water changes every two weeks results are 0.5 to 2.5 times weekly inputs very low for most of the time and a manageable peak level.
Last, I took a look at monthly water changes of 80%. This results in a range of 1 to 5 times weekly inputs. I do not recommend this, but the data shows that it is still FAR superior to weekly water changes of 20% for the majority of the month and the peak period is still the same even though you are taking the same amount of water out. So, for all but one day of the month your fish are swimming if much better quality water and the quality is effectively the same on the last day.
Bottom line small water changes are not sufficient, no matter how frequently they are performed. Large water changes are needed preferable frequently, but infrequent large water changes are superior to multiple small water changes.
Let me know your thoughts hopefully this helps,
Keith

Uptown Ruler
02-12-2014, 01:50 PM
I change 80-90% of the water in almost all tanks once a week......never lost a fish even some of the more sensitive species

peathenster
02-12-2014, 02:15 PM
I think Kevin did some simulations a few years ago, and posted some graphs.

I do the same as Derek, with the same results even for "sensitive" fish. Just drain the tank till the fish can barely stand up, then add prime and fill them up. Only thing is to make sure temperatures are not too different.

mountainman36
02-12-2014, 05:06 PM
i used to talk about this to folks years ago when i worked at several fish stores. if 100 bad units go into the tank every week, and you only take out 20% a week, its at 80, then the next week it is at 80 (from first week) plus the new 100 so its 180 for the next water change, 20% water change gets it down to 144....and it builds and builds.....

larger water changes mixed with smaller ones are still good, but i agree, large 70-90% changes get things going better.

I HAVE lost several fishes from large water changes.....my Congo tetras that were spawning in a community tank would go into shock....i killed half of them one water change, then did ok for a while remembering to be careful...then forgot and killed the rest of them. sucked because i was just about to move them to a separate tank to set up spawning...

obviously a good dechlorinator and some serious attention to temperature is important. That said, a few degrees cooler seems to be just fine. ive even accidentally gone from 81 down to 65 and the fish did ok. Infact its those big water changes that make a lot of fishes spawn.....most of you already knew that.


Thanks for the numbers and thoughts Kieth. I was always met with surprise when i tossed out those numbers of the water changes. glad to know im not the only crazy one.
Richard

ronv
02-12-2014, 06:01 PM
It seems to me that going to cooler water works better than going to warmer. Has anyone else seen that?

byork
02-13-2014, 06:57 AM
I think cooler water is better. I would love to see a chart or graph, some sort of visual illustration of your findings. My attention span is to short....plus I love charts.

BobMajor
02-13-2014, 08:11 AM
I dont believe there is any fixed rule on water changes.It depends on the species and whether the fish are new arrivals to your water parameters or have been accustomed to your water conditions for some time.With fish new to your water conditions small (20% ) and frequent water changes work best for me.fish that you have had for some time I do 50% water changes.These larger changes seem to encourage better eating and breeding.In the first instance I use water around the same temperature or slightly warmer to avoid the outbreak of ick and velvet to the new fish.in the second instance I use cooler water.I keep mostly smaller fish and many are wild imports so this is the system that works for me.Larger fish might be less sensitive be able to acomadate a wider range of variation in water changes

aXio
02-13-2014, 10:40 PM
Yeah I pretty much just drown my fish with water every week on the week.

Garfieldnfish
02-14-2014, 01:04 AM
I think you also need to take into account how heavily stocked your tank is (bio load and type of critters) and if you have live plants in it. I hardly change the water in my shrimp tanks. Maybe 10% every other week. The shrimp put out so little waste that the many plants just "eat" up the waste. If I were to do large water changes in those tanks I would most likely shock the shrimp and kill them. I have a 15 gal with 2 clown plecos and 3 ADFs. My dirtiest tank and I have been doing large water changes 60% to 70% weekly in that tank for years. Those fish/frogs are used to it and do fine. Most of my other tanks get water changes of roughly 25% every other week. But all of my tanks are heavily planted and need the fish waste as I do not use fertilizers. I personally prefer having live plants in the tanks to removing the waste via large water changes. I have several fish that are way older than their life expectancy, many of my fish are routinely breeding. I have had losses when doing large water changes, so unless there is a problem (like accidentally dropping half a container of fish food in the tank etc.) I will stick with the amounts above.

mountainman36
02-14-2014, 06:48 AM
plants would serve as a removal of waste products, just like the water changes, so 100 units in, 20 out from a 20% water change, plus ?10 units? out from plants.....yields 30 removed for a new total of 70.....obviously i just pulled those numbers, but it really doesnt matter if is 17 or 172 that i start with, either way its percentages.

I think the main thing to realize is that several large water changes, in excess of 50%, and in conjunction with smaller water changes are preferable in long term keeping of fishes to keep the waste levels down.

peathenster
02-14-2014, 11:32 AM
Found it. Post #3: http://www.atlantaaquarium.com/showthread.php?2052-Water-Change-150-gal

aXio
02-14-2014, 12:12 PM
Did you take into account the depletion of trace elements? A lot people tend to over look trace elements deplete-ting when water changes are stretched out to far. Also you have "units" of waste based on your own system. But did you take into account what filter the person is using, what filter media they are using, and how often they are cleaning this filter. Light duration/cycle (This doesn't grow just algae but promotes the growth of other micro organisms.). Another thing that comes into play is what type of substrate they are using and how much bacteria can that substrate support, along those same lines the age of the tank would also have to come into play because that usually correlates to bacteria levels. And to go even further is this test doing with gravel vacs or just a "pump and dump" water change?

Basically what I'm getting at is that in my opinion I think there are WAY too many variables to consider to make research like this viable at all. Even if you did a controlled test on a bare bottom tank with just fish and no filter, it still wouldn't translate to what is going in people's home aquariums.

peathenster
02-14-2014, 12:45 PM
Did you take into account the depletion of trace elements? A lot people tend to over look trace elements deplete-ting when water changes are stretched out to far. Also you have "units" of waste based on your own system. But did you take into account what filter the person is using, what filter media they are using, and how often they are cleaning this filter. Light duration/cycle (This doesn't grow just algae but promotes the growth of other micro organisms.). Another thing that comes into play is what type of substrate they are using and how much bacteria can that substrate support, along those same lines the age of the tank would also have to come into play because that usually correlates to bacteria levels. And to go even further is this test doing with gravel vacs or just a "pump and dump" water change?

Basically what I'm getting at is that in my opinion I think there are WAY too many variables to consider to make research like this viable at all. Even if you did a controlled test on a bare bottom tank with just fish and no filter, it still wouldn't translate to what is going in people's home aquariums.

I didn't take into account anything. That was not my post.

To your point though, I'm sure the weatherman doesn't know how many trees I have in my backyard (neither do I), but I still feel that the forecasts are informative.

hsd
02-14-2014, 03:23 PM
Found it. Post #3: http://www.atlantaaquarium.com/showthread.php?2052-Water-Change-150-gal

In that thread you linked, two good articles in TFH were mentioned. Nov and Dec of 2009 (Time for Change: A Mathematical Investigation of Water Changes, Part 1 and Part 2). Lots of graphs for data junkies and good explanations.

Garfieldnfish
02-15-2014, 09:44 PM
Take a 50 gal planted tank and add 5 large fish like goldfish or a couple of oscars and a large pleco. Now take these fish out and add 5 zebra danios and 1 oto.
I can see water changes being necessary for the large fish and the pleco but with the small fish and the oto, they won't even provide enough poop to keep the plants going. How can a blanket math chart for water changes work the same in both cases?

mountainman36
02-15-2014, 10:15 PM
Take a 50 gal planted tank and add 5 large fish like goldfish or a couple of oscars and a large pleco. Now take these fish out and add 5 zebra danios and 1 oto.
I can see water changes being necessary for the large fish and the pleco but with the small fish and the oto, they won't even provide enough poop to keep the plants going. How can a blanket math chart for water changes work the same in both cases?

your scenario illustrates it perfectly. If the goldfish etc create 50 units of waste and the plants remove 20 units, then a net 30 units are left every time period (could be a week or month)
The small fish likely create 10 units of waste (or less) and the plants need/remove the same 20 units....they can only have 10 as thats all there is, but there would be no net units left over for that set time period.....hope that makes sense.

you are right, every situation is different. The chart assumes that there is a net increase in waste every week, AFTER plants remove what they will and other biological stuff .....the specifics dont really matter, its just trying to show how a net increase in waste doesnt go down with only small water changes.

spidangular
02-15-2014, 10:50 PM
your scenario illustrates it perfectly. If the goldfish etc create 50 units of waste and the plants remove 20 units, then a net 30 units are left every time period (could be a week or month)
The small fish likely create 10 units of waste (or less) and the plants need/remove the same 20 units....they can only have 10 as thats all there is, but there would be no net units left over for that set time period.....hope that makes sense.

you are right, every situation is different. The chart assumes that there is a net increase in waste every week, AFTER plants remove what they will and other biological stuff .....the specifics dont really matter, its just trying to show how a net increase in waste doesnt go down with only small water changes.

I have to agree with the overall analysis and reply quoted above. The premise is based on the net units of waste being added to the closed system, so food added compared with waste removed, whether it be removed by plants or water changes.


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peathenster
02-15-2014, 11:07 PM
Like mentioned above, as long as there is net increase of waste, the trend should hold up. For lightly stocked tanks, the difference may not be significant enough.

canoe
02-16-2014, 06:12 AM
This is a mathematical tank we are talking about, not a real one, with assumptions about how much waste is produced, how frequent the water changes are, and the magnitude of each water change. When I was a biology major (long, long ago, in a distant place ... BPC*), we called that a simulation, based on a model (the assumptions). We used it to model an environment such as an aquarium over time. They are used all the time to gain insight into the dynamics of a system from economies to ecosystems to nuclear reactors. It is not, however, a recipe. The model provides useful insights about how the different variables interact, not exact answers. The fun part is increasing the complexity of the model, to bring the model closer to reality. Big fish, little fish, plants, filters, etc. are potential variables/complexities one could add to the model, but adding them would not change the original conclusions.

*BPC - before the PC

Garfieldnfish
02-16-2014, 07:41 AM
So my conclusion of the charts is that they are totally useless when it comes to tank maintenance.

aXio
02-16-2014, 08:11 AM
So my conclusion of the charts is that they are totally useless when it comes to tank maintenance.

I have to agree.

peathenster
02-16-2014, 10:41 AM
So my conclusion of the charts is that they are totally useless when it comes to tank maintenance.


I have to agree.

Now that's an interesting question. Does math become useless if you don't understand it, or refuse to try?

Back to the weather analogy, forecast says Monday high is 63F. It's not going to be 63.0000000000F by your house because of some local variables, so forecasts are useless? All the weather models are wrong?

If there are two shrimps in a 500g tank, sure one can ignore the difference. Hack, if you have eight 6" fish in a 120, you are free to ignore it too. I guess in that sense the charts are indeed useless TO YOU.

aXio
02-16-2014, 11:18 AM
Now that's an interesting question. Does math become useless if you don't understand it, or refuse to try?

Back to the weather analogy, forecast says Monday high is 63F. It's not going to be 63.0000000000F by your house because of some local variables, so forecasts are useless? All the weather models are wrong?

If there are two shrimps in a 500g tank, sure one can ignore the difference. Hack, if you have eight 6" fish in a 120, you are free to ignore it too. I guess in that sense the charts are indeed useless TO YOU.

I think the math you guys did is correct... and I do understand it very clearly thank you. I still don't think it is very useful in a tank maintenance situation. It useful to some extent but I think people would have to do an experiment such as this on their own tanks for it to be useful at all. Because in my opinion the variables I spoke of come into play WAY more then your taking into account. They make a lot more of a difference then your "weather man" analogy imo... although some people would argue that weather men are useless too =)

I'll have to keep siding with Sybille on this one sorry.

peathenster
02-16-2014, 11:46 AM
Just plug in any variables that you worry about into the formula, and you'll get your answer.

Not willing to use the tools and the tools being useless are two completely different things.

ronv
02-16-2014, 12:47 PM
In general, fewer large water changes do more to dilute waste than several small water changes. I think that is a fact backed up by the evidence. Deciding if that fact is necessary for your situation is the question.

Demonfish
02-16-2014, 12:58 PM
The change x % weekly forever strategy generally only works for 6-12 months even if you get the % right for the stocking. The specific numbers don't matter, a fixed input with a partial output will alway lead to stuff that builds over time. There is something called "old tank syndrome" where tanks that have been stable for years suddenly aren't.

I generally find that by the time a tank is in danger of crashing, I have a reason for a full breakdown anyway. The fish are big enough to move to bigger tank, I need to catch fry to sell, or I just need to move stuff around. Every time I get tank fish-free, I do a 100% water change and big tanks periodically but irregularly get big (down to fish laying sideways water level) water changes with substrate cleanings Small tanks often get their fish moved to a all-clean tank and then get dumped and cleaned.

How well fish take water changes depends on the fish. Some julies don't like it all. I have had "bad" water changes that had fish gasping at the surface from low-oxygen in the new water. That may be why cooler is better for some fish. I usually go a little warmer since there is all that evaporation from filling and filters waterfalling will cool it off and a sudden drop of temp can trigger ich. I advice people to start small and go up in %. If you have to do a big water change suddenly (the can of fish food in the water example fits here), just add the clean water back really slowly at first and speed up if the fish don't show any distress.

I do find planted tanks much more stable and can generally tell when they need attention by changes in the algae. The planted tank people don't change water to take out nitrate (there isn't any left), but to replace trace elements and they do huge changes to "reset" water parameters back to where they "belong".

joel03
04-27-2015, 11:36 PM
What are you guys using as a dechlor?


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