Tank size and Bioload dependence on PWC. But what about % ??

[kostasonia]

(a) I have heard that the smaller your tank is, the more amount of water you need to change in every PWC
(b) That more fishes means bigger PWCs too.

But reading about and percentages of water changed and frequency, I finally think that everyone has his own "homemade" rule about PWC.

For my 29g aquarium I do 10% every a week, and for the moment my nitrates keep under 25.

But thinking about it a bit more, the truth is that the less often you make changes (but of bigger percentage), the more of the contamined water you substitute.

For example. Is it the same to change 1.4% every day, 10% every week or 20% every 2 weeks?? If you do the last, 48,8% of the initial water will have been substituted after 6 weeks and if you go gor 1.4% every day, only 32.6%. Which is the best though?? Should small everyday changes be a bit bigger??

I wouldn´t like to focus the topic from the point of view that the more you last to do a PWC, the bigger the change abd therefore your system may loose stability, fishes stress etc. My question is:

How can you remove the maximum amount of polluted water by taking out the least amount of it (therefore disturbing the system the least)????

I hope the topic makes sense and that I gave you understand my question...

 

[joannde]

Hmmmm, interesting. However, I'm not sure this is a simple mathematical solution. If you do regular (weekly), large (50%) water changes then the water going out should be fairly close to the water going in in terms of pollutants (ie: since its only been a week there shouldn't be alot of pollutants, unless you are seriously overstocked or have ill maintained plants). SO, long story short - why not just change 50% per week ? in the "old days" where all we had available for water changes were buckets it would have been a large task, but with a Phyton regular large water changes are a breeze, and would seem to be best for both the fish and the plants.

 

[RogerMcAllen]

1.4% every day wouldn't really be enough to do anything, and waiting two weeks between changes might be too long. I'd shoot for something like 15-25% every week (in my case whichever it happens to be based on how close I can get with full 5 gallon buckets). By doing such a large change each week, your water doesn't really have too much time to change from what comes out of the tap. This in turn means no shock to the fish. As long as you arn't trying to alter your pH or something else, it shouldn't be a big deal. Also, it is good to get into a weekly habit of doing things so that you don't forget them.

 

[majolo]

I actually made a little spreadsheet about a week ago looking at this kind of question, just for curiosity's sake. You input the nitrate reading after a PWC, the reading right before the next PWC, the tank volume, volume changed, and the days between PWC. From that, it calculates the daily nitrate production and the eventual stable concentration range for the tank (since the nitrate levels will gradually rise in a new tank like mine until they are concentrated enough that the amount removed balances the amount produced). After that, in another column, you can play around with altering the frequency and size of the water changes and see the effect of it.

Here's a little example: Input:
First reading: 0ppm
Second reading: 5 ppm
Days between: 7
Tank volume: 208 liters (55 gal)
Water change volume: 52 l (25%)

Output:
Production: 148.6 mg/day
Stable concentration: 15-20.
Then if I tweak the PWC schedule, say I do 50% every other week instead of 25% weekly, it'll tell me that stable concentration will vary from 10 to 20 ppm. On the other hand, doing a daily water change of 7.43 l (which amount to 25% total over the course of a week) gives a range of 19.3-20 ppm, same max but smaller interval. I think these make sense, at least theoretically: when you reach equilibrium, you have to be removing the same amount of nitrates as are produced over a given time interval. So I think I would tend to disagree with the above post; if you change the same total amount of water per week, assuming good circulation in the tank, then daily changes should give a more stable water chemistry (with the same max nitrates) than weekly.

The model assumes a constant linear production of nitrates, 0 nitrates in the tap water, and perfect mixing (any of which may not be true). Also, you typically will not know the true volume of water in your tank (due to displacement by substrate, etc). Other than that, I think the main flaw with it is that nitrate test results for the inputs have pretty poor accuracy.

 

[fish_4_all]

It should make sense and as far as equilibrium, changing the water daily would prevent buildup even better. If I had the room to set up the water for my water changes and the desire to get the water ready for water changes doing 3 water changes a week at 25% each time wouldn't be a problem at all and my fish wold probably love it. I would say 10% daily would be even better for the fish and by using a spreadsheet, the dosing schedule and everything needed could easily be set up to fit the water changes.

My only thing would be my right arm would get really strong from lifting so many buckets through the week.

 

[DepotFish]

You need to do larger changes at once, not little changes daily. By changing the 1.4% daily and adding new water, the next day you are changing only 1% of the old water and .4% of the new water, the day after that you will take out .8% old water and .6% new water, and so on. By the end of the week you will have changed 9.8% of water, but only 5% will be over a week old. I hope I'm making sense. 10% is a small change anyway, not much difference to the nitrates.

 

[kostasonia]

The point is actually the nitrates. Someone could even setup an automatic water changing system where small portions of water automatically get thrown and new water replaces it. Ok, 1% or 2% per day is few. But the real question is if constant small changes are better than bigger changes.

What if someone could setup a system that changes a 0.5% every hour? This would suppose 3.5g changed water in a 29g tank every 24 hours or 24.36g per week. Someone can say that it is actually a waste of water if you can control nitrates with a 30% weekly changes. But isn't it what happens in nature actually? A river has a constant flow..

So I think that small water changes (that correspond to the same amount of contamined water changed) are better for keeping a really stable system. The problem is that an automatic water change is something like a dream for the moment.

 

[TwoHobbies]

I think Depotfish explained it better than I could have...reread his post..The answer to your question is No....small frequent (daily) water changes are not as beneficial as large weekly ones.

 

[Toirtis]

The point is actually the nitrates. Someone could even setup an automatic water changing system where small portions of water automatically get thrown and new water replaces it. Ok, 1% or 2% per day is few. But the real question is if constant small changes are better than bigger changes.

Yes. Imagine the benefit to your fish if a 5%-10% daily PWC kept nitrates consistantly below 10ppm, rather than permitting them to reach 20ppm, then doing a large PWC to lower them to say, 6ppm, then allowing them to rise again, doing a change, ad nauseum.

 

[kostasonia]

I think Depotfish explained it better than I could have...reread his post..The answer to your question is No....small frequent (daily) water changes are not as beneficial as large weekly ones.

the question is: 10% every day or 30% per week?? For my ease, 30% per week. For the fishes...??

 

[hc8719]

your schedule 1.4% daily or 10% weekly or 20% bi-weekly was a good idea, but after one week with no pwc, the water quality begins to downgrade even faster.

i dunno how cross country meets work in spanish high schools, in america we do 3.1 mile (5k)

if i run the first mile in 5 mins, you think "well if he can run that, that fast, than he can do 2 miles in 10 mins, or 3 miles in 15 mins. now some olympic runners could pull this off, i couldnt in high school. it slowly increase, 2 miles in 12 mins, 3.1 miles in 18 mins, and if you want to continue, 4 miles in 28 mins...

i dunno if this helps, but water quality is like a roller coaster, it may go steady for a bit, but once it hits a certain point, it drops...

 

[TwoHobbies]

I believe in a perfect setup ( everything in balance, plants and fish ) no water changes would be needed at all.

That being said, I still do weekly 40-50% water changes to reset my tank. I dose ferts, but if I thought my fish were at risk I would have no problem changing 10-30% daily.

I still agree with Depot, I just don't believe a "small" daily change is as beneficial.

Good thread..I'm interested to hear some more opinions

 

[majolo]

You need to do larger changes at once, not little changes daily. By changing the 1.4% daily and adding new water, the next day you are changing only 1% of the old water and .4% of the new water, the day after that you will take out .8% old water and .6% new water, and so on. By the end of the week you will have changed 9.8% of water, but only 5% will be over a week old. I hope I'm making sense. 10% is a small change anyway, not much difference to the nitrates.

First off, I agree that 10% weekly changes are on the low side. But I don't know where the rest of your numbers are coming from. Day 2, you're not changing 1% of the old water and .4% of the new water. Assuming a well-circulated tank, your are changing 1.4% of everything. But I really think this business of "old water" is a red herring (forgive the pun). Water doesn't age in any relevant sense (of course, I'm not claiming anyone literally thinks that). What water changes do is not "keep the water young," it is remove wastes. The more water you change, the more wastes you remove. The more frequently you change it (at a given rate of turnover), the stabler the level of wastes will be, with the same max concentration. That's my claim, and I can explain more about the model if anyone cares.

PS: It occurs to me that one other assumption that might affect the result is evaporation, which is not assumed in the model. But that's probably a small effect anyway. Also, the quote and italics aren't working in preview, hope this reads ok... Edit: apparently not. What happened?

Also, one effect that might distinguish between daily changes and weekly changes is that mixing will be less effective over one day than over one week, obviously. But that could go either way - you might remove water that has a higher concentration of wastes than the tank as a whole, or lower.

 

[kostasonia]

I really wonder about the change in water conditions after a 50% PWC. Apart from the nitrates, does anybody have a look if the rest of the parameters change too? Such changes should stress the fishes, no?? We speak about amount of minerals, pH, temperature, hardness.....Ok...if you have only tetras perhaps its easy. I admit that I do not have any experience on delicated fishes. But so many changes together!!

The whole question arised by reading that chocolate gouramis stress during every change of water...

And anyway...Even if someone thinks that weekly changes are better, still there must be an answer to the question. How much water should someone change every day in order to avoid bigger weekly changes?

 

[hashbaz]

I always find it nice to think of things in extremes.

Lets pretend we have a 100 Liter tank (26gal) and your fish poop 100mg of Nitrate per day (for simplicity we will ignore ammonia and nitrite). This means that your nitrate will increase by 1 ppm every day.

Next lets pretend that you randomly choose not to let your nitrates get above say 20ppm. There are an inifinite number of water change frequency and amount combinations that will comply with your goal.

Option A: One extreme is to do a 100% water change every 20 days.
Option B: Another extreme is to change 5 Liter of water every day.

Option A is pretty straight forward:

Option B (less straight forward) Every day before the PWC the water has 20ppm nitrate. After the PWC the water has 19ppm nitrate. The next day the water is back up to 20ppm and another water change is done. In each water change: 5L * 20ppm = 100mg (remember that the fish produce 100mg every day)

Results of option A:
Nitrates vary from 0 to 20
100 liters of water gets changed every 20 days

Results of option B:
Nitrates vary from 19 to 20
100 liters of water gets changed every 20 days

Conclusions:
1. Changing water more often does NOT require changing a larger total amount of water.
2. The percentage of change in nitrates is equal to the percentage of water changed.

An ideal system (in terms of least change in Nitrate levels) would be to change very, very small amounts of water very, very often. But this is not very practical.

In this discussion there are 3 variable: PWC frequency, PWC amount and ppm of nitrate. You can choose two and experiment to get the third. Example: I want 10ppm of nitrate and want to change water 1 time a week. I would then experiment with PWC amount until I get 10ppm of nitrate.

My suggestion: if you are only changing 10% every week you are only changing your nitrate levels by 10% while keeping your nitrates less than 25ppm. I do not see anything wrong whatsoever with your current practices. 1 PWC each week, 10% PWC and 25ppm are all very good numbers.

You have many different options:
1 PWC each week, 10%, 25ppm nitrate
2 PWC each week, 5%, 25ppm nitrate
2 PWC each week, 10%, 12.5ppm nitrate
2 PWC each week, 20%, 6.25ppm nitrate
1 PWC each week, 5%, 50ppm nitate
...and so on.

My final suggestion is to change your water 1 time each week on your day off (or some other day) and if you use a 5 gal bucket, fill it up as far as will not risk spilling.

 

[hashbaz]

I really wonder about the change in water conditions after a 50% PWC. Apart from the nitrates, does anybody have a look if the rest of the parameters change too? Such changes should stress the fishes, no?? We speak about amount of minerals, pH, temperature, hardness.....Ok...if you have only tetras perhaps its easy. I admit that I do not have any experience on delicated fishes. But so many changes together!!

The whole question arised by reading that chocolate gouramis stress during every change of water...

And anyway...Even if someone thinks that weekly changes are better, still there must be an answer to the question. How much water should someone change every day in order to avoid bigger weekly changes?

The amount the other parameters change will be the same percentage as the %PWC multiplied by the difference in paramters. (75° tank water - 70° tap water = 5° temp difference. 5° difference times 20% PWC = 1° change in tank temperature when you do a water change.) When concerned about paramters in your tank changing, more water changes are better. But your current practice of only a 10% water change will have little affect on water parameters.

Do chocolate gouramis stress becsause of changes in water parameters, or do they stress because they freek out when you stick your hand and a hose in the tank?

 

[majolo]

Just wanted to say, Hashbaz' post above is exactly what I was talking about, and a much clearer example. Thanks for posting it.

 

[hashbaz]

Just wanted to say, Hashbaz' post above is exactly what I was talking about, and a much clearer example. Thanks for posting it.

I don't know if I would agree that my example was more clear - but thanks anyway.

I'm a big time nerd, so I love thinking about this stuff.

I liked the way you brought up equilibrium because this is key to the discussion. No matter how often or what quantity you change your water - it will reach an equilibrium point, and from then on every water change removed water with the equalirium level of nitrates.

(Of course this is ignoring things like vacuuming poop during water changes, siphoning out ammonia and nitrate before it becomes nitrate, and variations in tank conditions due to bioload and feedings and a host of other things)