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The weekly 50% water change.... why?

Epiphyte

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I've had this nagging question in the back of my mind for some time now, I'm hoping the combined wisdom of UKAPS may help me make better sense of it.

Convention says that with a high tech aquascape, I must perform a 50% water change weekly, which I do, give or take a day or so. My question is though... why?

If I look at my tank, it's roughly 160 litres of RO water (Oase 90P), remineralised to a TDS of 120. It has large amounts of plants, mostly fast growing stem type. Fish stocking for the aquarium size would probably be classified as low, roughly 30 small tetras, a dwarf gourami pair and a flotilla of Red Cherries. I lean dose APT Complete and the whole thing gets turned over with an Oase Biomaster Thermo 600, filled up with excess amounts of Seachem Matrix. None of my hardscape choices affect water quality or chemistry either. For all intents and purposes, the water is immaculate and, minus a small amount of BBA on the hardscape, is completely algae free.

So why with a well maintained and mature aquarium, with plants exporting so many of the nutrients and so little coming from waste, do we do this unusually large water change? If I get to the 7 day mark, everything bad reads zero and nitrates are low, what am I actually exporting via the water change?

I'm not expecting there to be double blind peer reviewed studies over this, but anyone I have ever asked sort of shrugs and says they aren't sure why we really do it. Are we doing it because it's convention? Because there is something in the water we don't traditionally measure that we're exporting? Or am I just pouring perfectly fine water down the sink every week?

I appreciate that when something is going wrong, a big water change is the first thing to do, but this is all assuming a well operating tank.

Interested to hear peoples thoughts.
 

Ajm200

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If you look back at some of the older Estimative Index posts you should find your answer. There’s lots of scientific talk that explains it properly

When I first joined the forum I was just starting in the hobby. The advice was to overdose fertiliser to ensure that there was enough of everything, lots of flow and lots of CO2 then a 50% water change at the weekend.

The result for me was a rapidly growing tank with plants that needed lots of maintenance, algae problems and sometimes gasping fish as I didn’t have the experience to balance a tank like that.

I went very low tech. Some of my tanks are only filtered with plants above and below the water. I have dirt with a gravel cap, low light, no filter, lots of plants and a few fish. I add a ferts if the floating plants look like they need it. The water gets topped up and changed if it is looking a bit yellow. It’s a little ecosystem I can balance as I’ve removed most of the human factors. I have no visible algae but had a daphnia population explosion that had to be brought under control
 
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MichaelJ

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Hi @Epiphyte, Ok, I'll take a stab at this first... Im running low-tech tanks and NEVER did CO2, so keep that in mind. First off, we do water changes to get rid of waste from fish, uneaten food, plants, algae spores and pathogens etc.. H2O is H2O... if we didn't have any unwanted by-products we could more or less just top off evaporated water and replenish fertilizers thats been eaten by the plants. Now, if your tank is mature (which it is as you state) and have found a balance and is seemingly stable you might possibly be able to get away with less frequent water changes or less amount. A good approach might be to see what sort of drift you measure in terms of TDS (not everything will show as TDS, but a lot of by-products will). If you remineralize to 120 ppm and see no large drift between water changes it might suggest that waste creation is roughly cancelled by uptake... You still want to get rid of that waste though. One suggestion would be to lean on some measurement such as TDS and use your eyes and good common sense - we can usually tell from the spirit of the plants and livestock if we are falling behind on maintenance. The fact of the matter is that WC frequency and amount is not cut in stone - its super tank-depended on specifics such as stocking level vs. plant density etc. Speaking from my own experience I went from 50-60% weekly to 35% every other week (I try to keep it 12 days in-between but it often end up being 14-15 days) and see no difference - if I go really long, like 5 weeks that I had to do over the late spring this year it definitely shows in a bad way - big spike in TDS, some lackluster plant appearance, visible detritus buildup ... For new tanks regardless of tech the consensus around here is generally to suggest at least 50% weekly - more for high metabolism tanks - but again, as the tank matures and plants grow in you might be able to tweak that to far less.

Cheers,
Michael
 
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Yugang

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all assuming a well operating tank
In this case only (partial) EI dosing (or other overdosing) needs big weekly water change. This is because the reset is an essential part of EI to work without poisoning plant and lifestock with the excess amounts of ferts (accumulation).

Convention says that with a high tech aquascape, I must perform a 50% water change weekly
So this is not true, even not for high tech. If ferts dosing is low, and healthy tank, less water change can work fine
 

MichaelJ

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In this case only (partial) EI dosing (or other overdosing) needs big weekly water change. This is because the reset is an essential part of EI to work without poisoning plant and lifestock with the excess amounts of ferts (accumulation).

Hi @Yugang, True. However, I never understood the rationale behind this reset- part of the Estimative Index approach as it doesn't make much - if any - mathematical sense. No matter what amounts of fertilizer you are dosing (within reason), you will always reach some sort of equilibrium (where there is no additional accumulation) with your dosing and a sensible WC regime - and if your following EI you are already buying into an approach that is less concerned about "overdosing" :)

Cheers,
Michael
 

Yugang

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it doesn't make much - if any - mathematical sense
Actually it does, you can play with the nutrient accumulator to simulate

Nutrient Accumulation Calculator​


Just to clarify the idea, let's take two extreme examples.
  • No plants and no water change. After a couple of years the ferts concentrations in the tank will approach the concentrations of the dosing stock solution.
  • 100% water change weekly. We know after each water change and dosing exactly the ppm in the tank and it will be same every week. This is what EI aims to achieve.

The basic idea of EI is that we don't know how much the plants consume, and therefore we don't know how much ferts we would have to add to keep the tank ppm constant. With a 50% water change reset, the accumulation can never exceed a factor of two above the target for the tank - irrespective of the plant consumption. As we now have limited our maximum accumulation to this factor of 2, we can safely overdose (dose more than plants need). This overdosing is the idea behind EI, so that we can rule out deficiencies.
 

GreggZ

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Don't change your water for a couple of months and see how it goes. That's a great way to learn.

Water changes are easily the best thing you can do for your planted tank. If you really want to whip it into shape do a couple of week for a few weeks.

It has more to do with removing organics than anything else. An uber clean tank makes every single other thing easier. No question about that.
 

Yugang

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Don't change your water for a couple of months and see how it goes. That's a great way to learn.

Water changes are easily the best thing you can do for your planted tank. If you really want to whip it into shape do a couple of week for a few weeks.
This

It has more to do with removing organics than anything else.
The point you make for removing organics is clear, but for an EI dosed tank the accumulation of ferts (no waterchange, while continuing to dose) seems an even bigger issue than organics. Of course risks are less when the EI doing has been carefully reduced, and one would hope that we are closer to a balance between dosing and plant consumption.
 

Epiphyte

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Lots of interesting reading so far, I'm certainly not looking to avoid water changes for months, I'm just conscious on waste water and wasted time. I completely understand with EI you're doing it to avoid "saturating" the water column with ferts over time. However, is this still the case with lean dosing?

Absolutely understand the idea to clean organics out of the tank, but I could definitely do a good vacuum of the plants and substrates with maybe 1-2 25L buckets.
 

KirstyF

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I think the amount of organics being released in a perfectly healthy high tech tank with a few fishes might still be more than most think! and cleanliness is next to…awesomeness!

There is however (like with most things) a range.

The guys who remove every aquasoil bobble and polish it….at least I’m sure that’s the only way they can get tanks THAT clean. (you know who you are. 😉)

And those who merrily trot along on less W/C volumes or frequencies and have very happy tanks.

But, conventional wisdom is conventional for a reason. It covers all bases, should be more than enough to keep most tanks ticking along nicely and certainly isn’t going to do any harm (mostly, quite the opposite)

Can you do it differently, sure you can. You would need to consider ferts accumulation else your lean APT might not be so lean after a while, and you’d need to figure out what levels worked for your tank in your situation through experimenting…..be prepared for the odd nasty surprise to pop up cos things can happen fast in a high tech! (where did all that algae come from 😱) …..and observe the tank super closely so that it can tell you how you’re doing.

If all that sounds like fun then go for it.
If the desire to reduce is pressing (time pressure, wanting to reduce water water from RO etc) then consider it.
If you you want to keep it simple….50% WC will do that for you. 😊
 

tam

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The 50% and maintaining an equilibrium with the estimative index makes sense, but seven days is also quite an arbitrary number - probably picked because a lot of people water change on the same day of the week. I imagine, you probably wouldn't see a difference if you water changed every 10 days instead, which would save you 16 water changes a year. At which point, I'd start wondering if every two weeks would suffice if your plant growth and numbers looked fine.

But, I also don't do high tech, so I'm more of a 20% every few weeks.
 

LMuhlen

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Another positive influence of this general rule of changing 50% water weekly is to help destroy an old lasting myth that excess water change is bad, that we shouldn't change more than 10% to avoid hurting the balance of the system. I'm not sure how widespread this myth is over there, but I do hear it from time to time, and having it as a general rule to change at least 50% water on tanks is a great way of showing to these people that they shouldn't be afraid to change water.

The way I see it, general rules are meant to provide enough safety margin to prevent as many issues as possible for the widest audience possible. But if you want to optimize your system and are willing to experiment a bit, there is nothing wrong with finding what works for you. However, some of the issues with reduced water changes may be subtle and not immediate, so it is important to keep a sharp eye to avoid having your tank slowly deteriorate in an unnoticed way.
 

GreggZ

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The point you make for removing organics is clear, but for an EI dosed tank the accumulation of ferts (no waterchange, while continuing to dose) seems an even bigger issue than organics. Of course risks are less when the EI doing has been carefully reduced, and one would hope that we are closer to a balance between dosing and plant consumption.
Yes this important as well. But it's often misunderstood. Folks need to understand how water changes frequency/amount affect water column levels of nutrients.

There is a formula to calculate the theoretical maximum accumulation of nutrients. It's the dosing between water changes divided by the water change percentage.

So let's say someone doses 15 NO3 between water changes. That can mean a lot of things depending on the water percentage.

For instance:

15 ppm NO3 with 25% water change: 15/0.25 = 60 ppm max accumulation
15 ppm NO3 with 50% water change: 15/0.50 = 30 ppm max accumulation
15 ppm NO3 with 75% water change: 15/0.25 = 20 ppm max accumulation

Same dosing, same tank, just different water change percentage. If folks don't understand maximum accumulation they should. In the end we are trying to maintain a steady level of nutrients in the water column.
 
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hypnogogia

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15 ppm NO3 with 25% water change: 15/0.25 = 60 ppm max accumulation
15 ppm NO3 with 25% water change: 15/0.50 = 30 ppm max accumulation
15 ppm NO3 with 25% water change: 15/0.25 = 20 ppm max accumulation
shouldn't that be:
15 ppm NO3 with 25% water change: 15/0.25 = 60 ppm max accumulation
15 ppm NO3 with 50% water change: 15/0.50 = 30 ppm max accumulation
15 ppm NO3 with 75% water change: 15/0.75 = 20 ppm max accumulation
 

MichaelJ

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It has more to do with removing organics than anything else. An uber clean tank makes every single other thing easier. No question about that
@GreggZ that is uber true :) It's just that what it takes to maintain that uber cleanness vary by the overall metabolism ( organic waste generation) of our tanks. For instance, my tanks can easily get by with 35% every two weeks (I try to keep it 11-12 days in-between) and they are very clean as far as I can tell. Someone else may need 60-75% per week to maintain the same level of cleanness... So thats that.

I guess my dosing approach is slightly out of tune with what people are normally doing as I add all my ferts (NPK/Ca/Mg) except for traces, to my WC water (targeted relative to the WC amount only), as opposed to dosing throughout the period in-between WCs. It just makes the whole thing so much easier and consistent for me.

I'm just conscious on waste water and wasted time.
Me too and thats why I slowly went from doing +50% weekly to 35% every 11-12 days....

I completely understand with EI you're doing it to avoid "saturating" the water column with ferts over time. However, is this still the case with lean dosing?
I guess it depends on how you apply your dosing... I am rather lean with my dosing as well and do not have any noteworthy accumulation.

There is however (like with most things) a range.
Very much agree!

but seven days is also quite an arbitrary number
Blame it on the Babylonians - possibly!
that we shouldn't change more than 10% to avoid hurting the balance of the system
Yes, that is generally considered an old wife's tale... However, I can imagine scenarios where the parameters of the water added would vary wildly from the tank water parameters and cause instability (stress on the livestock in particular). As long as we avoid that, large water changes wont cause problems.

Cheers,
Michael
 
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GreggZ

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shouldn't that be:
15 ppm NO3 with 25% water change: 15/0.25 = 60 ppm max accumulation
15 ppm NO3 with 50% water change: 15/0.50 = 30 ppm max accumulation
15 ppm NO3 with 75% water change: 15/0.75 = 20 ppm max accumulation
Good eye I changed it above pressed submit too quick!!!!
 

Tim Harrison

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There's these ancient threads too with Tom Barrs (@plantbrain) input...;)


 

KirstyF

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guess my dosing approach is slightly out of tune with what people are normally doing as I add all my ferts (NPK/Ca/Mg) except for traces, to my WC water (targeted relative to the WC amount only), as opposed to dosing throughout the period in-between WCs. It just makes the whole thing so much easier and consistent for me.

Curious about this.
So, if you are adding ferts to WC water at a specified level, we would assume that the water in the tank has been depleted to some extent by uptake, so your starting point moves progressively downwards. Your top up ppm is consistent so doesn’t compensate for that, unless your top up ppm is, in itself, enough for the tank, even if the remaining water were at 0, which would, effectively prevent any potential for an underdose!

i.e the amount of ferts in the top up water is adequate for the tank as a whole regardless of uptake.

Is it this technical or would you just splosh in a bit extra if it ever looked like it needed it? (Which can be an equally effective method if you know what to look for 😊)

Also, how do you set ur micro target? Do you pick a total target dose and then simply divide it into however many doses you will be putting in over the WC gap and does this influence the target number you choose or the number of doses, as each individual dose would be smaller, the more doses it is split into.

i.e more smaller doses? or typical 1/3 dose but more days apart?

Also, if the micro target was a total based on full water volume and not just WC water, would that then not accumulate?

Sorry for all the questions but you’ve got my head ticking now! 🤔😂
 

Yugang

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Folks need to understand how water changes frequency/amount affect water column levels of nutrients.

There is a formula to calculate the theoretical maximum accumulation of nutrients. It's the dosing between water changes divided by the water change percentage.

you can play with the nutrient accumulator to simulate

1669340563747.png
 
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