This is a short post.
I see a lot of aquarists afraid to do a 100% water change because the rule against it is so deeply ingrained in the hobby.
Let me say this right away. Sometimes you HAVE to do a 100% water change.
Okay, so let's dive into this. Larger water changes can result in bigger swings in parameters, true, and so you don't want to do too big or too many just because you would be unnecessarily stressing out your live stock.
That is fine and fair. But! It is a false belief to think that a 100% should absolutely be avoided at all costs. Why? Simple.
What's worse than stressing out your live stock? Them dying.
If there's a chance that your water is causing your fish to die, do a 100% water change. Why not smaller? What about a 50% water change? A 50% water change will only get rid of 50% of the problem (if you want to think of it that way). Why not 75%?
Well do you think 75% or 80% or 90% is going to be THAT much different compared to a 100% water change?
Nah.
If you are in danger of losing your fish, do a 100% water change. Causing them short term stress is better than letting them swim in very bad water conditions.
Examples when you might want to do a 100% water change?
You extremely overdosed on medication. Here I will give some concession to smaller water changes, if you did not overdose by much. But well, when people overdose, it can quite often be more than by double.
You accidentally poured soap or some other harmful chemical in the tank. Even a small amount can kill your live stock, so yeah.
Your ammonia is off the charts. Just do a 100% water change, please. With the API test kit, 8ppm is its max, and even if it was at exactly 8ppm, a 50% water change takes that down to 4ppm. Still high. A 90% water change takes it down to 0.8ppm. Not too bad, but STILL HIGH. And that's if it's actually 8ppm. Off the charts mean that - off the charts. It could be 20ppm for all you know. Then, a 90% water change would only take that down to 2ppm. And so on.