water change

[fish.]

can changing too much water or changing water too often kill fish?
(specifically when water parameters change dramatically for the better)

 

[Kolykaf]

If the water you are using has parameters not radically different from that which you are removing I wouldn’t be too concerned. If it’s very different, it could definitely impact the fish’s well being. For example, say water changes haven’t been done on a tank for a very long time and tds has risen considerably. The fish may be fine even though the level is outside their optimal range because the levels have gone up slowly and they’ve become accustomed to it. Then, a big water change is done that reduces tds substantially. This can cause the fish to go into osmotic shock and kill them.

 

[Circus]

If a tank is in particularly bad shape (think a year plus of no water changes or something similar) you would want to gradually do very small water changes, as sudden or large changes can kill fish.

 

[Byron]

The article on water changes may provide more data.

Regular Partial Water Changes

Byron Hosking November, 2010 (rev April 2021) Nothing is more important when it comes to aquarium maintenance than regular partial water changes. The adage “an ounce of prevention is worth a pound of cure” certainly applies here. One frequently reads of products that will reduce the need for...

www.fishforums.net

"Parameters" and "conditions" are two very different things. Parameters are GH, KH, pH and temperature, and provided these are reasonably close between tank water and tap water, the more water changed the better. However, if maintenance has been inadequate, or the tank is overcrowded, or being overfed, etc, then ammonia may be a concern depending upon the pH.

 

[P J]

Hi mr fish, aim for 10% to 15% water change a week - but some with heavy planting may reduce that figure.

 

[Spyro]

can changing too much water or changing water too often kill fish?
(specifically when water parameters change dramatically for the better)

Yes it can.
I found out the hard way that tap water is full of minerals/stuff that might be harmful to your fish and/or lacking minerals/stuff that your fish needs.
The water you are adding might have perfect general parameters (PH, Ammonia, Nitrates, etc) and appear to be super healthy but in reality you are adding oil to the fire sort of speak: your fish is already stressed from being in water with not so perfect general parameters and you just added harmful chemicals/minerals with water change. That takes a while for carbon to remove or something. But your general parameters don't test for any of that.

For example:
-When I added fish after cycling tank, the ammonia/nitrites get a little bit out of wack with feeding, etc.
-Fish is already stressed a bit.
-I have a planted aquarium.
-Plants use oxygen during the night thus water is oxygen low in the morning.

-I get up on Saturday morning and decide to clean the tank and change water. I do 50% water change adding crystal clear water with good PH, no ammonia, nitrates, etc. and replacing dirty greenish water with traces of ammonia, nitrites and fair bit of Nitrates.

Good thing?

It turns out not for me or my fish.
I just removed a lot of algae and N. bacteria that removes ammonia by changing water, vacuuming substrate and cleaning glass.
That will cause ammonia to spike a bit.
Furthermore, unknown to me: my tap water has no oxygen in it at all and trace amounts of harmful chemicals/minerals.
So basically:
My Saturday morning water change did this:
-Removed half of already low oxygen from the tank leaving fish with no oxygen
-Removed fair bit of algae and N. bacteria causing ammonia to spike
-Added trace amounts of harmful minerals/chemicals to the tank
-Stressed fish by sticking hand and vacuum into tank
-Stressed fish by changing water parameters a bit.

So even do I improved general water parameters by changing water I'd have few fish die withing a day or 2 after water change.

It turns out everyone has different conditions, tap water, etc so one glove fits all with water changes doesn't always work.
If you have fish dying after water changes like me: you'll have to figure out why on your specific setup and conditions.