Water changes

sammydee

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I've been thinking about water changes and I can't really get my head around one little fact.

For example, you change 25% of your water every week, and lose 5% every week by evaporation. That means you have to put 30% of water back into the tank, right? So surely, the minerals etc in your tank would slowly be getting more and more concentrated. So eventually, the water will just get harder and harder, with more and more chlorine or trace elements in it. Have I missed something? Is this right? It just worries me a bit that I might be slowly but surely increasing the levels of toxins in my tank until one day they get too much for the fish and become poisonous.

Can somebody explain this to me please? Am I right?

Thanks
 
Yea, there's been an arguement like that going around for some time now. Personally, I don't know what to believe alhough I must admit it makes sense, all that stuff building up in your tank. I think it's called Old Tank Syndrome, or is that a term used for something else???
 
So what can you do about it unless you top up with distilled water?

What really confuses me is that I know that if you take (for example) 95% of the water out and replace i, the minerals will be diluted, yet if you take 5% of your water out and replace it, it will become more concentrated over time. This confuses me a lot!

Any help?
 
I think you are assuming the minerals in the 25% of water you remove stay behind. The water you remove may be concentrated by the 5% since it has evaporated but the water you syphon off will not leave its minerals in the remaining tank water. so it would seem that it would take much longer to concentrate to dangerous levels.
I don't know if doing a 50% change every couple times would help since you would remove half of the concentrated minerals in the tank with a 50% change and slow the build-up process by half since you removed half of the concentrated water.

Think of it this way... your tank has 100 gallons
5 gallons evaporate
the 95 gallons have 5% more minerals
you syphon off 20 more gallons
leaving 75 gallons
you then add 25 gallons of new water
your tank has 100 gallons of water with roughly 3.75% more minerals (5% with 25% dilution)


This is getting too confusing :blink:

itZme
 
The problem here is that when you syphon out water, you syphone minreals and such with it. water that evaporates leaves everything behind, thus making the water harder (it has more minerals)

topping off with RO water, like people with marine systems do, would fix this issue. But just doing 25% water changes every week seems sufficient.

I don't think 5% of my water evaporates every week, maybe like 2%. If I had no cover, it would most likely be more but anyway...

let's calculate this

for this example, let's say you have 10000 mineral particles in 100 litres and 5% evaporation every week

after 1 week, I have 95% of the volume left (95 litres) and 10000 minerals.

I will remove 20 litres and add back 25, thus a 25% water change
I remove 20 litres on 95 total.

10000 particles for 95 litres = 2105 for 20 litres

so now we have 75 litres of water left with 7895 particles
I top if off with 25 litres of tap water (2500 particles)
100 litres (10395 particles)

that's a 3% increase every week with a 25% water change and 5% evaporation

10395 week 1
10706 week 2
11028 week 3
11358 week 4

after a month, we would have increased hardness by 13.5%

this is ridiculous so perhaps I have screwed up my calculations or most likely, minerals are absorbed by plants, evaporate after a longer period of time, are absorbed by fish, bacteria....
 
But it must happen. I think that if you change over 50% of the water, you can bring the mineral concentration down, but anything less than that pulls it up.

I'll do a hypothetical experiment for a 20% water change each week and see what happens.
100 minerals in a 100 gallon tank to make it easier to work out.
Assuming a 5% per week evaporation (to make it easier, assume no minerals evaporate from tank or leave tank in any other way):

Week 1 - Take out 20%, replace 25% so there are 105 minerals.

Week 2 - Take out 20%, replace 25% so there are 110.25 minerals.

Week 3 - Take out 20%, replace 25% so there are 115.7625 minerals.

Week 4 - Take out 20%, replace 25% so there are 121.550625 minerals.

The minerals can't evaporate I'm sure - maybe some are taken up by plants but some people don't have plants and unless you trim the leaves they can't just disapear. We are talking about ALL trace elements in the water here, too, not just minerals. There are lots of things we cant measure.

The trace element count CLEARLY must increase - so what happens?

Lets say the minerals have increased until they reach 150% of the original - so a 100 gallon tank has 150 minerals in it.

Now we do a 20% water change - take out 20 gallons, so we are left with 120 minerals in 80 gallons. So, we replace 20% of the water - but the new water has 100 minerals per 100 gallons. So we put 20 minerals back.

Now the mineral level is 140!!! It has DECREASED!!!

There must be a limit between 121.550625 and 140 that the minerals reach so that it starts decreasing again, unless my maths is flawed (not a typo - in Britain we say MATHS not maths).

I am even more confused now - common sense tells me that the trace elements must increase, but the maths clearly says that it has decreased!

Anyone got the maths skills to figure this out. I will continue the weekly water change hypothesis when I have more time.

Any help would be nice I'm very confused about this :crazy: .
 
ok thats not confusing!!

quite simple......make sure you don't have any evapuration then there isn't a problem.

Alot of minerals get "used up" so don't exist anymore in the tank, thats 1 of the reasons why you have to do water changes to replace the minerals in the water needed by the fish.
 
This is a differential equation problem.

To make it much more simple. Assuming nothing is used up by fish/plants/bacteria and that only water is evaporated.

Don't look at it as concentration of minerals, try to think about it as total number of minerals.

Your total number of minerals will continue to increase until the number leaving the tank (dirty water) = the number entering the tank (clean water).

At this point you reach equilibrium. You cannot take the argument to the extreme and say each water change puts X amount of minerals more in, because after awhile the amount in = the amount out. Basically, I say, don't worry too much about it and if you really want to, every so often buy some distilled or RO water and do a water change with that.

I hope this clears it up for some people.
 
Yes you are right paul_v_biker, but it isn't just minerals that is the issue. Other trace elements that we don't know about like some heavy metals or maybe chloramine or whatever that isn't removed by commercial make-water-safe additives could build up and become harmful even if they aren't harmful at first.

Yet this is clearly NOT happening. My tank is over two years old and in all that time I haven't had such a dangerous build up of chemicals. So what exactly is going on here?

I went over my maths in the hypothetical four weeks and it is wrong. The four weeks should do this:

(Again assuming a 100 gallon tank, with one "mineral" (mineral being a loose term for all the trace elements in the water) per gallon and 5% of water left to evaporate every week)

Week one: 20% removed, 25% added - 105 minerals
Week two: 20% removed, 25% added - 109 minerals
Week three: 20% removed, 25% added - 112.2 mineral
Week four: 20% removed, 25% added - 114.76 minerals

The mineral level is still increasing. I'll continue with the series to see what happens.

Week five: 20% removed, 25% added - 116.808 minerals
Week six - 20% removed, 25% added - 118.4464 minerals

(the rate of increase is definately decreasing - which is a good thing - hopefully it should stabilise soon)

Week seven - 20% removed, 25% added - 119.75712 minerals
Week eight - 20% removed, 25% added - 120.805696 minerals
Week nine - 20% removed, 25% added - 121.6445568 minerals

The rate of increase is decreasing more and more. Hopefully it should stop pretty soon. Lets skip ahead to see what happens if the "mineral" level is at 130:

Week X - minerals at 130. 20% removed, 25% added - 129 minerals (!!!)

So there we have a decrease. I think I am starting at last to understand why this happens as well.

Basically, when the minerals in the tank are more concentrated (as the water evaporates more and more over a long period of water changes) when you remove a certain amount of tank water, it has more minerals in it per gallon than tap water. So when this concentration of minerals hits a certain amount (which could be more or less depending on how much water you change - if you change less water, you will get more mineral build up) they equalize.

In the hypothetical case I tried above, this golden concentration seems to be about 130% of the tap water. However, this would clearly be smaller if there was less evaporation.

So, I think that because the evaporation per week is 5%, and this is a fifth of the 25% water change you make, the total "mineral" concentration will end up as a fifth more than your tap water IE 125% of your tap water. We can test this easily:

Week X - Mineral concentration at 125. 20% removed, 25% added - 125 minerals

So this is the golden concentration.

Hence, the formula for working out the extra amount of trace elements that would be present in your tank eventually, and what concentration (as a percentage of the concentration in your tap water) that they would stabilise at is this:

Let E = Percentage of your tank water that evaporates every week.
Let C = The percentage of your tank water that you replace every week.
Let A = Concentration of trace elements tank water would stabilise at.

A = (E/C)*100 + 100.

Its so simple! Why didn't I realise this before? So, using this formula, you can work out how much water you have to change to get the tank water as close as you want to tap water.

Therefore the build up of tracve elements is NOT RELATED to old tank syndrome unless the owner had high evaporation rates and changes the water rarely.


Basically for those less mathematically minded, if you change 25% or more of the water a week, you don't really have to worry about it at all.

I hope I didn't bore anyone stupid! It's made me slightly less confused at least anyway.

Thanks for your help!
 
The minerals that control the carbonate hardness and general hardness in the water are used up constantly by organic acids that are released during the nitrogen cycle so you have to do water changes to replenish the buffers (minerals) before they reach a level low enough for the pH to crash.
This will happen far faster than a overload of minerals in the tank causing the pH KH and GH to rise and is a guarentee rather then a theory.
If you are suffering large losses through evapouration then you will need to top the tank off with R/O or distilled water.
 
There's no way that you could lose 5% of your water in short amount of time, if you do regular water changes like once a week you won't even notice it...Just make sure you have a lid or a canopy on your tank....
 

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