Can fish survive with 1 half tank of old water and 1 half tank of new, clean water?

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My younger brother asked me this question, and I don't know the answer.

Will a fish survive with 1 half tank of its old, gotten-used-to water and 1 half tank of new, clean water?

img_1522010_0_8228a3e6ee3662dc29fd796b39aee659.png
 
Yes providing you treat the tap water before hand. Even so 1 water change shouldnt kill any fish
 
Is this supposed to be a trick question?

anyways, yes.

I don't see how its any different from doing a 50% water change.
 
My younger brother asked me this question, and I don't know the answer.

Will a fish survive with 1 half tank of its old, gotten-used-to water and 1 half tank of new, clean water?

img_1522030_0_8228a3e6ee3662dc29fd796b39aee659.png

I believe that is what's known as a 50% PWC ... so the answer is YES.
Did you remember to dechlorinate the new water.:facepalm:?
 
Upon further thought ... the main question to ask is .. how did the original water get to the 1/2 point?.

1. If you removed it and replaced it with the new (hopefully treated water) then that's a 50% PWC and the answer is ... YES.

2. Did 1/2 the original water evaporate away? If so, then all nitrates, gases and other impurities are still in the water ... but more concentrated. In that case adding clean water is not "really" a 50% PWC. You will end up with the same original concentration as when the tank was full. In this case, it's probably best to follow up with a proper 50% PWC.
 
I don't know how the water got to half of the tank's total volume. And unfortunately, I don't have anything that is useful to treat tap water within a short time span.

And these are good answers for my brother. Thanks.
 
If you haven't got any additive to remove the chlorine and chloramines then you can leave the water to stand...

If you pour our the water you want to put back into you tank into clean (detergent free!) buckets/bowls and leave them for 24hours then a lot of the chlorine will gas off. Chloramines will still be present, but it's better than just dumping fresh tap water into the tank.

If you can run airstones in the water as it's standing this will help the whole process along by agitating the water..
 
Can simple, nonstop water agitation help reduce chloramines or chlorine?

Chlorine will naturally degrade out of tap water rather quickly ... agitation could help it off gas as it promotes gas exchange. Chloramines is more tricky ... doesn't degrade as fast I believe.

Can you get a bottle of dechlorinator like Seachem Prime? It deals with chlorine & chloramines practically instantaneously.
 
I can't. I couldn't find any stores. There's a thread with more info about my situation. I am using Android to type, so I cannot link to it.

EDIT:
Now that I'm back at home, I can properly provide the link, here.
 
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2. Did 1/2 the original water evaporate away? If so, then all nitrates, gases and other impurities are still in the water ... but more concentrated. In that case adding clean water is not "really" a 50% PWC. You will end up with the same original concentration as when the tank was full. In this case, it's probably best to follow up with a proper 50% PWC.

I disagree. If that amount of water evaporated away, those levels are going to be toxically high but the fish have gotten used to them. If you reduce those levels by half (by filling it) and then half again (by the water change) in a very short time span, you'll end up throwing the fish into shock which will likely kill them. When a fish tank has been neglected like that (this hypothetical situation, not the OPs specific situation), in my experience it's best to start with very small water changes every day or every other day as to not shock the fish. I also wouldn't fill a half empty due to evaporation tank in one shot.
But that's just my opinion.
 
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