Are Large Water Changes Killing Fish? Small vs Large Water Changes -

tarheel96

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Some people believe performing large water changes can hurt or even kill their fish. I've even seen threads where it's also been suggested that large water changes can harm or kill the beneficial bacteria. So, only a small amount of water is changed at a time (10 - 30% max).

Others believe that large water changes do not harm fish and they perform fin-level water changes or more (75 - 90+%). They drain the tank as low as possible so they can change as much water as possible at once. The size of the water change is mainly limited by the size/height of the fish.

What do you believe? Can large water changes harm fish? Can they kill the beneficial bacteria? If so, why? I hope we can reach consensus on this and also about why there are two schools of thought.
 
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Angelphish

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Nitrates aren't completely removed unless you remove all the water in the tank and start with fresh water, because, assuming you feed your fish the same amount each week, they will produce the same amount of nitrate between each water change.

As an example, if your tank makes 20ppm of nitrate weekly, doing a 50% water will dilute the nitrate to 10ppm. The next week, before you change the water, the nitrate is at 30ppm. This is because it was 10ppm at the beginning of the week, and then the fish added another 20ppm by the end of the week. This will always happen unless you change all the water. Larger waterchanges only delay the rise in nitrate.



The equation for dilution is (C₁ x V₁) = (C₂ x V₂)

C₁ = Concentration before dilution, in this instance nitrate concentration.

V₁ = Volume before dilution, but for water changes you use the amount of water left after the water is removed but before it's refilled (if you had a 100 gallon tank, and you did a 20% water change, then 80 would be plugged into V₁. Whatever % of the water you change from your tank, you convert it into a 2 digit decimal (20% = 0.20, 64% = 0.64. etc...), subtract it from 1, then plug the difference into V₁. For example, .95 would be used for a 5% water change, .63 would be used for a 37% water change, etc...).

C₂ = Final concentration of nitrate, after the water change is done. This is the variable that's being solved for.

V₂ = Final volume, the amount of water in the tank after the water change.



If you do 30% water changes weekly, and your fish create 20ppm of nitrate each week, then your nitrate would be 14 after the first week, 23.8 after the secondweek, then 30.66, 35.462, 38.8234, etc... By the time it hits 45 nitrates (just in this instance, not for every tank), it slows down to +0.1ppm per week. Although it may plateau at 45ppm, this is still quite a high amount,especially for a tank with a good water change schedule. Most people only do weekly water changes, and usually weekly water changes result in a higher nitrate plateau.

(C₁ x V₁) = (C₂ x V₂)

(20 x 84) = (C₂ x 120)

(1680) = (C₂ x 120)

(1680) = (C₂ x 120)
-----------------------
120

(14) = (C₂ x 1)

14 = C₂


Smaller water changes done more often are more efficient than large weekly water changes, as long as each water change is 1/3 the amount you would do weekly. For example, if you did 10% water changes every day on a tank that produced 20ppm of nitrate weekly, something that would take less than 5 minutes for the average sized aquarium, the nitrate would be 13.4ppm rather than 14ppm. Although this change may seem insignificant, it reduces the nitrate plateau to 25ppm from 45ppm.

The absolute most efficient water change schedule is the weekly amount changed daily. Even as little as a 30% water change done daily on a tank that produces 12ppm of nitrate per day, or 84 per week, can keep the nitrate plateau at 30ppm, which is incredibly low for atank that creates such a high amount of nitrate.
Something I posted in a thread a while back.
 

Chockful O Phail

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Large changes can harm if your water is unstable. Also if temp is too much higher, but only with certain fish. Most can easily handle a cooler change, my daughter recently was helping me and got it too hot by 8*~, not many survived.
If you have stable water and can temp well large changes do no harm.
 
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jaws7777

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I guess there are instances where large water changes would cause harm depending on the stock or a possible swing in ph beteeen tap and tank water but for me fin level is the way to go. I even drain most of my sumps. Been doing this for a few yrs now and never had an issue.
 

jaws7777

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For a bit of anecdotal evidence, aside from the heat massacre, I’ve done weekly fin level+ for 4+ years and no visible harm has come from it. There have been several -10+ degree changes in there when I ran out of hot water in the winter.
Funny you mention that but i get the impression my guys enjoy the cooler water. They all get more active and swim in the flow a little more vigorously
 
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Fish Tank Travis

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I guess there are instances where large water changes would cause harm depending on the stock or a possible swing in ph beteeen tap and tank water but for me fin level is the way to go. I even drain most of my sumps. Been doing this for a few yrs now and never had an issue.
I do this too. I let the pumps pump the water up into my tank while I'm draining it so that I get as close to 100% as possible.

IMG_2436.JPG

This is what they get every weekend. Sometimes I can get it even lower than this.
 

Steve_C

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Count me in with the large WC group as well. All my tanks from my 240g down to my 29g and everything in between gets 75%-85% done every 8-10 days. The majority of the BB in a tank lives in the filter media itself, the rest in the substrate and decor. Hardly any BB at all is actually in the water itself. I usually fill the tank with about 4-5f cooler water and the fish are always happy. Been doing it this way for many years.
 
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