Why a weekly 50% WC is better than two weekly 25% WCs

[calioutlaw1a]

Many hobbyists perform a 25% water change per week, a 50% water change per week, or some do multiple water changes per week (25% water change two times per week in this example).

One of the primary reasons for performing water changes is to maintain a low nitrate level. When considering nitrate levels, it is obvious why a weekly 50% water change is better than a weekly 25% water change, but it isn't immediately intuitive as to why a weekly 50% water change is better than two 25% water changes per week.

A weekly 50% water change is superior for two main reasons:
1. Nitrate levels reach equilibrium faster (this is minor)
2. Our fish are exposed to a lower average nitrate level each week.

Compare the following data:
(Edit: I apologize but the tables I tried to paste did not format correctly, so just the results are posted. If anyone is interested I can try to post up the data tables)

Scenario 1: Starting at 20ppm nitrate, the nitrate increases 20ppm per week (or 10ppm every 3.5 days). A 25% water change is performed twice per week (every 3.5 days).

Equilibrium is reached where after each 25% water change the nitrate drops to 30ppm, and after 3.5 days it increases to 40ppm.
So at equilibrium:

• Minimum nitrate = 30ppm
• Maximum nitrate = 40ppm
• Average nitrate = 35ppm

Scenario 2: Starting at 20ppm nitrate, the nitrate increases 20ppm per week. A 50% water change is performed each week.In this case, equilibrium is reached immediately, where nitrate is 20ppm after each 50% water change and increases to 40ppm at the end of each week.
So at equilibrium:

• Minimum nitrate = 20ppm
• Maximum nitrate = 40ppm
• Average nitrate = 30ppm

The same amount of water is changed each week, and the maximum nitrate each week is the same for both, but the minimum nitrate is always lower with a 50%wc (in any scenario) resulting in a lower average nitrate per week.
I know this is a lot for what may seem trivial or not very important for many, but I know some, like myself, appreciate data/numbers/evidence for what we do and why we do it. Chronically elevated nitrate levels are now starting to be implicated in some fish diseases, and we are just now starting to have some decent research on nitrate and its role in aquarium health.
For anyone interested, I may be posting some data on recent nitrate research coming from Dr. Floyd at the University of Florida and info from Dr. Weber at the UC Davis School of Veterinary Medicine.

 

[MonsterMinis]

If your Max nitrate for the given week didn't exceed 40ppm I would agree.. but generally doing 25% WC's 2-3 x a week is to keep them below 40PPM steadily.. just as other tank perameters are monitored. it's generally understood consistant, more even water quality is more important then "perfect" conditions that fluctuat to much.

Thats the only flaw i see in your logic.. but Its deffinately worth knowing, and understanding why some tanks should have more water changes then others, and why some tanks don't "Need" excessive abouts of water changes.

The only other flaw I guess I see and a minor one at this point is the build-up of Organics and other things generally we don't test for that are being suspected more and more of causieng damage/disease in our pets. Which are often seen in conjunction with elevated nitrates.

 

[calioutlaw1a]

If your Max nitrate for the given week didn't exceed 40ppm I would agree.. but generally doing 25% WC's 2-3 x a week is to keep them below 40PPM steadily.. just as other tank perameters are monitored. it's generally understood consistant, more even water quality is more important then "perfect" conditions that fluctuat to much.

When considering water parameters such as pH, alkalinity, hardness, temperature, or salinity consistency is key. However, we are talking about a substance that is harmful to our fish, and thus reducing it as much as possible is the aim. I'm not aware of any evidence suggesting that fluctuating nitrate levels is harmful to fish, but I am aware of evidence suggesting that higher nitrate levels are bad for our fish.

The only other flaw I guess I see and a minor one at this point is the build-up of Organics and other things generally we don't test for that are being suspected more and more of causieng damage/disease in our pets. Which are often seen in conjunction with elevated nitrates.

I should have added the qualifier that these scenarios are assuming all else is equal. There are many other variables to consider, but all else being equal 50% weekly WCs are superior to two 25% weekly WCs.

 

[MonsterMinis]

The answer to your first quiry.. "Old tank syndrome" which granted takes more into account then simply nitrate levels but likely dissolved organics as well. It is often seen in tanks that are "topped off" but never get actual water changes that the fish exposed to a significant water change sometimes as little as 25% will suddenly keel over and die though the water perameters read absolutely fine, except the nitrate levels. Extreme changes in otherwise "stable" environmenets even if considered unhealthy often maintain fish that otherwise "should be dead". This is extreme cases generally but to what levels do Nitrates become this toxic? and is it the Nitrates alone that cause this toxicity, or something else?

What you are suggesting though intersting on a scientific level really needs to be more closely scrutinized on a realistic level imho. I'm certainly interested in the data but call me skeptical that these conditions can not be met 'nore readily acheived in the home. To boldly state a flat statement like that but not include the other variables included in Nitrate build-up is borderline silly. Hobbyists don't get the option to "omit" organic build up ect as part of the by-product that creates Nitrates in our tanks.

I don't think the question that Nitrates alone in large amounts are detrimental to our fish, It's a fairly simple conclusion comparing it to other life forms of similar composition ( un-proven, but a perfectly logical conclusion). Just as ammonia, or nitrites are. The question is how much is it impacted by the other by-products that come with it. So stateing flatly that 50% WC's are better then 2x 25% a week as you use for refernece is a fact-less and misleading statement to make, it doesn't include the other variables that can and most likely do play a role.

Your tossing out fairly simple numbers, and equations. Which is deffinately not such a bad thing, But until it is backed by hard facts... It is a Conclusion... .. interesting... But a Hyphothesis, Not a Fact.

 

[calioutlaw1a]

The answer to your first quiry.. "Old tank syndrome" which granted takes more into account then simply nitrate levels but likely dissolved organics as well. It is often seen in tanks that are "topped off" but never get actual water changes that the fish exposed to a significant water change sometimes as little as 25% will suddenly keel over and die though the water perameters read absolutely fine, except the nitrate levels. Extreme changes in otherwise "stable" environmenets even if considered unhealthy often maintain fish that otherwise "should be dead". This is extreme cases generally but to what levels do Nitrates become this toxic? and is it the Nitrates alone that cause this toxicity, or something else?

What? Old tank syndrome is a term that describes the result of long-term neglect of a tank resulting in the exhaustion of the buffering capacity (alkalinity) and subsequent drop in pH, leading to death of the biofilter and increase in ammonia. It is this increase in ammonia that makes even small water changes potentially lethal because with a water change the pH will increase and more ammonium will shift to ammonia.
I would imagine the nitrate levels would also be high in a neglected tank, but this is not what old tank syndrome is referring to.

What you are suggesting though intersting on a scientific level really needs to be more closely scrutinized on a realistic level imho. I'm certainly interested in the data but call me skeptical that these conditions can not be met 'nore readily acheived in the home. To boldly state a flat statement like that but not include the other variables included in Nitrate build-up is borderline silly. Hobbyists don't get the option to "omit" organic build up ect as part of the by-product that creates Nitrates in our tanks.

The scenarios I gave are reasonable (and in fact quite common). If you have 2 equivalent tanks where the vast majority of organic debris is collected in a filter (which is the case for many on this site), then there is no organic debris to clean up with a WC (ie vacuuming). Further, my scenarios don't distinguish between sources of nitrate. One tank could have debris, both could have debris, or neither could have debris. The question then is which is more important: Reducing nitrates or removing organic debris. I would argue the former is more important based on what we see in natural environments (low nitrates and lots of organic debris). For the hobbyist, vacuuming the tank bottom is often more for aesthetics than health.

 

[Calihawk]

Uh if I may, Thank you Calioutlaw1a for your info. We will all take this information you've given us into knowledge that we'll keep.

I may not need to do that type of WC because I only have 1 Flowerhorn and a pair of Jewels, and 8 frys in a 120 gallon. But I found that sometimes if I take my time siphoning ALL the corners and spaces btw that the WC becomes closer to 50% anyways. Good INFO though

 

[dayak]

whoa this is good post, thank you calioutlaw, since i have been here i could count with my fingers out of million posts thats been posted since i have been here.
and its make sense and no question about it your scenerios if everything is being equal
i do weekly WC about 30-40% of water depands how heavy the poops i have to vacuum LOL

 

[dogofwar]

If only most people who keep aquariums did EITHER one 50% water change per week...or two 25% ones! I'd guess that 90% do not....probably even monthly.

If you're going to do two 25% water changes per week, why not just make one of them 40-50%? A few minutes extra but worth it.

Also, I'm a fan of mixing in a large (75%) water change every month or so to reduce nitrate and other waste in the tank to near the levels from the tap.

Matt

 

[jlnguyen74]

Your theory/argument/reasoning is wrong. I'm not even bother with data, and number, and just go with common sense and nature observation. While you're focus on the small picture of nitrate level, you're missing the big picture of mineral depletion, pH level, and hormone released by fish in to aquarium water. Thus, small amount of water change at more often frequency is better than large amount of water change at less frequency. Look at nature. Fish lives in river, stream, where water flows constantly from high ground to lower ground. At any location, you have constantly/frequently water change, supply by cleaner water from high ground that flows thru that point. In summary, a 50% weekly water change won't be better than two 25% water change per week.

 

[dayak]

well seems like we debating on the right thing both are right none of them wrong.
does it matter if one do water change 30 and 20% twice a week, 25% twice a week or 50% a week or even 75% a week, some will do fine with monthly
the result i bet on they all will have good water all the time for their given condition and parameter