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Water changes-different schools of thought


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Hi Guys,

I am a large water changer (not me, the water lol). I do 60-70% water changes twice a week. I've been doing this for years and the fish seem to love it. I have heard the argument that if you do more than 25% water changes at a time, you can "shock" the fish. Just wondering what people's views are on this. I've done 100% water changes before when there have been problems. No ill effects. I know most beneficial bacteria are on the surfaces of the tank/filter/decorations, not in the water, so it can't be about interrupting a cycle....New water will replenish the oxygen, also a good thing, right? Maybe someone could explain to me how clean water can adversely affect fish:)

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Firstly, I think you already have the answer from the way your fish responds. :)

Anyway, to answer your question, I think that the people who recommend 25% or less at any WC:

- may be making this recommendation with tropicals in mind. Certain tropical fish are exquisitely sensitive to changes in the water, and could be stressed out. With goldfish this is certainly not the case, and 100% WCs, bucket to bucket, etc don't faze them, but can be helpful in times of sickness.

- may be making this recommendation in lieu of making sure that tap and tank waters don't differ that drastically. Large water changes are fine, but only provided that you match temp, and pH. For some people, there can be a big (0.5-1 or more) difference in pH, and with that, large water changes without matching the pH are not recommended.

- may be making this recommendation because they don't keep goldfish. Adult goldfish put out a lot of waste, and even if you are going with 20 gallons/fish, I don't think that a 25% WC weekly, or bi-weekly, would be sufficient to keep nitrates below 20-40ppm at all times. :)

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I agree with Alex. Some tropicals are stupid sensitive to changes in the water. And if you keep sensitive fish who at the same time won't produce the same mess that goldfish can, you need to sometimes take it easy with the waterchanges. Make it more gradual and not chock them to death.

I also think some people don't like doing water changes. And will want to do as little as possible. With tropicals (and light stocking) you can get away with 25%/week.

And some people assume that all fish are the same. And as such, make the same recommendations for how to keep them.

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For more than a year, I had a school of glowlight tetras in with my goldfish. They did OK together, but I noticed that the tetras did NOT like the big water changes. They were much more sensitive to the change in temp, I suspect. Although the temperature ends up the same by the time the tank is filled, it's not uncommon that it fluctuates as I'm filling it. I lost one once because the water fell a couple of degrees for a few minutes.

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I always do 80-90% every week. I've had people tell me I'm killing my fish and every time you change more than 25% you have to go through the acclimation process again .-.

I agree that these people are applying the rules for more sensitive tropicals

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Since I went from keeping tropicals to keeping goldfish I still haven't been able to get rid of my fear of doing too large waterchanges. I know I've killed fish with waterchanges before (unfortunatly, those fish were also my mother's but the larger waterchanges were 100% neccessary that time). With the super sensitive ones, it might not matter much that you try your best to match the ph, temp etc. Sometimes they just can't take the stress of other minor changes in water chemistry. But, I have adjusted so I won't go crazy when doing 50% on my goldfish tank. So far, that's enough seeing as they're so utterly tiny. But I will have to get used to doing larger and larger changes. And I can tell you, I think it's scary. :blink:

I guess you get used to one way of doing things. I would just never do more than 30% on a tropical tank unless there was an emergency. I'd rather do 20% every day for a few weeks than do more than 30% at once. But with the goldfish, I assume more at once is better than a little here and a little there.

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For more than a year, I had a school of glowlight tetras in with my goldfish. They did OK together, but I noticed that the tetras did NOT like the big water changes. They were much more sensitive to the change in temp, I suspect. Although the temperature ends up the same by the time the tank is filled, it's not uncommon that it fluctuates as I'm filling it. I lost one once because the water fell a couple of degrees for a few minutes.

Yes, many tropical fish are much more sensitive to this sort of thing. There is a reason that it is recommended to change the temperature on a tropical tank only a couple degrees over the course of hours. A sudden change of a couple degrees within a few minutes can sometimes affect a weaker individual.

I also think that it may not only be the change in water chemistry that may harm these little guys, but the overall stress of the disturbance itself.

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I've never kept tropical fish. Well,my daughter has a betta (she is 5 and does all of the maintenance with my supervision, so proud lol) who gets 50% water changes weekly. Maybe I should go to 25% twice a week.

Makes sense to me. Her betta does get a little nervous when we are working on the tank. The goldies look at it like it is time to munch on whatever I can rustle up for them:) Often I have to move them away from the siphon. I'm glad I get within 0.2 degrees of the tank temp.

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Since I went from keeping tropicals to keeping goldfish I still haven't been able to get rid of my fear of doing too large waterchanges. I know I've killed fish with waterchanges before (unfortunatly, those fish were also my mother's but the larger waterchanges were 100% neccessary that time). With the super sensitive ones, it might not matter much that you try your best to match the ph, temp etc. Sometimes they just can't take the stress of other minor changes in water chemistry. But, I have adjusted so I won't go crazy when doing 50% on my goldfish tank. So far, that's enough seeing as they're so utterly tiny. But I will have to get used to doing larger and larger changes. And I can tell you, I think it's scary. :blink:

I guess you get used to one way of doing things. I would just never do more than 30% on a tropical tank unless there was an emergency. I'd rather do 20% every day for a few weeks than do more than 30% at once. But with the goldfish, I assume more at once is better than a little here and a little there.

Thanks for your response. It's interesting to see how it is for someone who has kept both types of aquarium. I'd be scared too lol. I feel guilty if I do less than 50% twice a week...

I always do 80-90% every week. I've had people tell me I'm killing my fish and every time you change more than 25% you have to go through the acclimation process again .-.

I agree that these people are applying the rules for more sensitive tropicals

Okay, good! I'm in the same boat for the most part. I was afraid I was going too far.

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It's not just people who have tropicals. On pond forums, you have people -- usually long time ponders -- who say one should never do more than a 25% water change, since more will destroy your ecosystem. They are full of stories of people who lost all of their koi after a large water change.

Fortunately, even the old fossils are switching to continuous water changes, which produce steady-state conditions in the pond.

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It's not just people who have tropicals. On pond forums, you have people -- usually long time ponders -- who say one should never do more than a 25% water change, since more will destroy your ecosystem. They are full of stories of people who lost all of their koi after a large water change.

Fortunately, even the old fossils are switching to continuous water changes, which produce steady-state conditions in the pond.

The thing that needs to be distinguished here is that this comment is directed at ponds, where the system, even with overstocking can still withstand a lot more nitrate production, because it's a more complete and robust ecosystem.

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Why would a more robust ecosystem be disrupted by a water change more than 25%? I'm talking about advice given to someone who has sick fish and bad parameters being told he brought on the problem by doing a 50% water change.

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Why would a more robust ecosystem be disrupted by a water change more than 25%? I'm talking about advice given to someone who has sick fish and bad parameters being told he brought on the problem by doing a 50% water change.

I assumed that you were talking about ponds, but I am mistaken.

In that case, then the ones who say that 25% or greater WCs will disrupt the ecosystem makes no sense to me.

Moreover, this makes even less sense to me when it comes to sick fish. We routinely do 100% WCs, or bucket to bucket, and that works out rather well for the fish. :)

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Firstly, I think you already have the answer from the way your fish responds. :)

Anyway, to answer your question, I think that the people who recommend 25% or less at any WC:

- may be making this recommendation with tropicals in mind. Certain tropical fish are exquisitely sensitive to changes in the water, and could be stressed out. With goldfish this is certainly not the case, and 100% WCs, bucket to bucket, etc don't faze them, but can be helpful in times of sickness.

- may be making this recommendation in lieu of making sure that tap and tank waters don't differ that drastically. Large water changes are fine, but only provided that you match temp, and pH. For some people, there can be a big (0.5-1 or more) difference in pH, and with that, large water changes without matching the pH are not recommended.

- may be making this recommendation because they don't keep goldfish. Adult goldfish put out a lot of waste, and even if you are going with 20 gallons/fish, I don't think that a 25% WC weekly, or bi-weekly, would be sufficient to keep nitrates below 20-40ppm at all times. :)

You missed one Alex:

- Misinformation :(

I was told by my lfs that the bacteria are in the water column and if I change too much water I will destroy my cycle :(

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Why would a more robust ecosystem be disrupted by a water change more than 25%? I'm talking about advice given to someone who has sick fish and bad parameters being told he brought on the problem by doing a 50% water change.

I assumed that you were talking about ponds, but I am mistaken.

In that case, then the ones who say that 25% or greater WCs will disrupt the ecosystem makes no sense to me.

Moreover, this makes even less sense to me when it comes to sick fish. We routinely do 100% WCs, or bucket to bucket, and that works out rather well for the fish. :)

I was talking about ponds. I don't think either of us is understanding the other. It makes no sense to me either, but it is a very common opinion among pond people.

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Well when speaking about ponds. If part of your ecosystem is balanced do too water suspended algae then removing too much of the water could negatively affect the ecosystem. But right off hand that is the only thing I can think of that would cause a severe problem.

Sent from my SCH-I535 using Tapatalk 2

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Firstly, I think you already have the answer from the way your fish responds. :)

Anyway, to answer your question, I think that the people who recommend 25% or less at any WC:

- may be making this recommendation with tropicals in mind. Certain tropical fish are exquisitely sensitive to changes in the water, and could be stressed out. With goldfish this is certainly not the case, and 100% WCs, bucket to bucket, etc don't faze them, but can be helpful in times of sickness.

- may be making this recommendation in lieu of making sure that tap and tank waters don't differ that drastically. Large water changes are fine, but only provided that you match temp, and pH. For some people, there can be a big (0.5-1 or more) difference in pH, and with that, large water changes without matching the pH are not recommended.

- may be making this recommendation because they don't keep goldfish. Adult goldfish put out a lot of waste, and even if you are going with 20 gallons/fish, I don't think that a 25% WC weekly, or bi-weekly, would be sufficient to keep nitrates below 20-40ppm at all times. :)

You missed one Alex:

- Misinformation :(

I was told by my lfs that the bacteria are in the water column and if I change too much water I will destroy my cycle :(

Linzi, I was told this last week by the new guy at the store. I was telling him how I am treating for Ich and flukes. When he said it, 2 other employees looked over with these "oh man, new guy sucks" faces. I explained it to him. He was very embarrassed. lol. I get tired of bad pet store advice. So dangerous.

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Well when speaking about ponds. If part of your ecosystem is balanced do too water suspended algae then removing too much of the water could negatively affect the ecosystem. But right off hand that is the only thing I can think of that would cause a severe problem.

When I think about it, ponds frequently diverge quite a bit from the tap pH. Furthermore, one fills a pond from the hose, so the incoming water might be quite different in temperature from the pond water. Neither is likely to be a problem for goldfish, but koi can respond very negatively to environmental shocks.

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There is also the issue of the cost of a koi, some of which can be in the thousands or even more. In that circumstance, I would certainly go with the path of minimal change that can still permit excellent conditions. :)

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