How much is too much?

[Timberwolf]

I am having cycling problems in my 37-gallon tank because I had to tear down my 18 gallon before I was fully ready to. In the last week I have lost some of my favorite fish and I am scrambling to try to get back ahead of it with large volume water changes.

To avoid stiring up my substrate as much as possible, I drain off a bucket full and put a fresh bucket of well water straight back in and repeat the process. By doing this, I am able to change larger volumes of water without depriving the fish of too much swimming area, refill the tank without clouding it to opacity, and reduce the potential of osmotic shock.

My question is a simple one:

Can I do too great a water change, and what are the potential risks?

[Bas Pels]

Sometimes it is possible to change too much. for instance if your nitrate is very high, and you would change 95 % in one time, your fishes would have to cope with too large a change

But I never heard of any problem as a result of 75 %.

Generally speaking, it is better to do 2 changes of 75 % each on 2 consecutive days than one of 90 %

But if you know something is wrong with the water, 90 % - or more - could be a good idea.

Taking the fish out into a bucket of clean fresh water would be a 100 % change, and sometimes this is the only solution

If cycling did not work, daily waterchanges of 50 to 75 % for a fortnight migh be a good idea

[MatsP]

There is only one problem with large water changes: Sudden and large change of water parameters - e.g. large pH changes, large changes in TDS (roughly the same as hardness), etc . This is nearly always only a problem if the tank is heavily polluted or has gone quite long without water changes. If your tank is newly set up, that wouldn't be a problem - unless you filled it with hard tap water, and then do large water changes with very soft water (e.g. RO or shop-bought distilled water).

I regularly do 50% water changes on most of my tanks - the 4 tanks that get the RO waste water have a continous run of fresh water, which makes about 80% water change per day.

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Mats

[sunfish]

The way you do a water change is actually quite inefficient and for most fish there's no need to do it like that. When you do a water change the only difference between what goes out of the tank and what goes into the tank should be "polluted" vs. "clean". And unless you haven't changed the water for months (or keep very delicate fish) even changing 90% should not cause problems. And a little less room to swim in for a very limited time should not be a problem either. One very important parameter that many disregard is water temperature. When you do a large water change, don't use cold water.

In order to avoid stirring up your substrate cover it with a saucer, a plastic bag or something else before you pour in fresh water. If that's not possible, break the current with your hand.

There's one thing that can ause serious problems, though. Straight from the tap water usually contains a lot of dissolved gas. If you put large amounts of water all at once straight from the tap into the tank this can actually lead to aeroembolism in your fish, killing them. But since you're using water from a well, this is a problem you don't have.

[MatsP]

Tina,

Very good points. It is certainly no problem taking half or a bit more water out of the tank, and then replace it with the same amount of new water - and it has a bigger effect on the pollution you will remove. If we, say, take out 10% of the water, then pour the same amount back in (and it gets perfectly mixed), the next time you will remove 10% of the water, but 1% of the water taken out will be water that you just put in. Next time around, you will take out a little less than 2% of the new water you previously put in. And as you go along, you are actually taking out a diluted amount of the pollution.

To make this obvious to yourself, try this:
Take a cup of coffee [1], and a same size cup again. Take out two tablespoons of liquid, and put it in second cup, replace it with water. Keep going until you have a full cup in second cup.

Now do the same thing, but poor half a cup into second cup, fill first cup up with water, and do a second half a cup.

You fill find that the first cup is darker and the second cup is lighter in the first experiment - indicating that there is more pollution (coffee). It may not be a huge difference - but I think you would see a difference.

[1] Or any other dark coloured drink - black-currant/black grape juice, wine, etc will also work for this purpose. Or some water colour or something like that.

--
Mats

[2wheelsx2]

If you change the water straight from the tap, it's best to splash it around (aerate it) well if you're using a bucket, and if you are doing a large tank and using a hose straight from that tap, splash it off the glass to get as much of the dissolved gas out as possible and you shouldn't have this problem.

[Shane]

Can I do too great a water change, and what are the potential risks?

The basic answer is that under normal circumstances no you can't.

As Bas and Mats pointed out above. If a tank is really far gone a large water change can cause stress. This is because of mainly two issues: 1) the fishes will have adapted to the very high pollutant levels and 2) high pollutant levels and infrequent water changes can alter the water's chemistry (esp pH). A large water change on a filthy unmaintained tank can in fact knock the fishes off pretty quick.
I once was asked by a boss' wife (since I was a fish guy) to take care of her tank while she was on vacation. Since her husband was paying my salary, and I was a broke college student, I immediately agreed to do so.
It was the nightmare tank we have all seen. A 55 gallon with garish pink gravel housing a large oscar, a large Hypostomus and a large doradid. "Decor" was a couple of ceramic decorations right out of Sponge Bob's world. Opening the tank's top I was hit by that "tank gone bad" smell. As a side note, I have realized that over the years my nose has become a pretty good indicator of a tank's water quality.
I doubt the tank had ever received a proper water change and was probably just topped off now and again when evaporation brought the water level low enough to make the filter too loud.
This is the kind of tank where a 50% or more water change would probably drop the animals dead.

If a tank is healthy, well filtered and fairly clean a large (70-80%) water change will have zero impact on the fishes. Obviously the new water needs to be within a couple of degrees of the old and the chemistry should be similar, but this is not a problem for 90% of aquarists as their tank water is their tap water anyway so the chemistry will be almost the exact same.

I also agree that there is no reason to take and then replace each bucket. This is a lot of extra work and, as Mats example shows, somewhat counterproductive. Your fish will be find in 3-4" of water while you finish the water change.
-Shane

[2wheelsx2]

I also agree that there is no reason to take and then replace each bucket. This is a lot of extra work and, as Mats example shows, somewhat counterproductive. Your fish will be find in 3-4" of water while you finish the water change.
-Shane

I agree completely.

There are discus keepers with bare bottom tanks who change enough water their discus have to lie flat when they do a water change. Your fish are not going to mind at all as long as you follow the guidelines the others have posted above.