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Why water changes? (was Re: [APD] pH shock)

Robert Zink <bonsai at hrcreditunion_net> came up with:

Thus spake revance at indiana_edu on 4/1/2004 1:37 PM:

IMHO drastic changes of any kind are not healthy for the fish.

That seems to make a lot of sense from a common-sense point of view.

So if this is true, I ask: why all this talk about 20% water changes, 50%
water changes, 80% water changes...  ? And why so regularly?

If your water tests are OK, why do so much water changing?
-- Thanks, Bob (a newbie prepared to be edified) : )

Edification Mode <ON>

The statement isn't exactly correct. Fish and plants tolerate considerable change as long as it is within proper bounds and non-toxic. For example, light may change a huge amount every day, and only *lack* of that change is harmful! [Don't try to deprive fish or plants of their circadian rhythm.]

That's one of the evils of mythology like pH shock. It leads one to look for the wrong things.

Fish and plants don't tolerate well the build up of toxic metabolites and loss of essential trace elements that will gradually happen in any active living system. For the most part, those cannot be measured by anything but fish and plant health. One way to compensate is to regularly add fresh, active water (not DI, RO or other "dead" water) and replace some of the older, less tolerable, water.

It is important to have the new water match the osmicity (tds level) and temperature level of the old water to some degree. Swings of tds of much more than a factor of two (particularly if downward) are hazardous, and many fish get upset at more than 5 degrees of sudden chilling (while others are just invigorated by a cold shower of 20 degrees or so).

Neither fish nor plants are bothered by sudden changes of 3 or more full points in pH, so worrying about that alone just enriches the LFS with little benefit to the tank. pH-change simply does not tell you what you need to know. A conductivity (tds) meter does, and your finger easily can feel a difference of 5 degrees. Those are the two most important tests, far and away.

Measuring GH and KH are a good idea, too, for if the GH gets too low, sodium (as in salt) can become rather toxic to fish and, particularly, plants. Low KH can make the pH so unstable it can go low enough that the CO2 can cause blood acidosis -- aka "pH crash." Fish and plant metabolites tend to be acidic, and some pH buffer (KH) in their water can protect their bodily fluids from that acidosis.

In the latter example, measuring pH is too indirect, as KH is the important parameter and can be measured and adjusted upward with a bit of baking soda. It should be about 4 degrees or more. I feel 2 or more degrees of GH is pretty good insurance against salt poisoning (from food, etc.)

Measuring ammonium/ammonia is a bit questionable, for the amount of ammonia that is known to be toxic is down around 5 ppb -- yes, parts per billion! The conversion from ammonium (fairly harmless plant food) to ammonia (really poisonous) does not start upward until pH is about 6.5. Any amount you can measure with the lousy LFS kits is going to be harmful at any pH much above 7. To make that guess you need to measure *both* ammonium and pH. I don't bother, and just make sure I have enough active photosynthesis and/or biofilter going that ammonium would always measure at zero on the typical test. When in doubt, I'll add a bit of "Amquel" o/e to tie up the ammonium until the plants can use it. [That's mostly just for shipping fish when the other ammonium sponges are not effective.]

Fish and plants need some changes, and are irritated by others. Most good aquarists eventually learn which factors do what. Hope this helps.


Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net
                    760 872-3995

Eschew obfuscation and bloviation!

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