What part of chlorine harms fish?

[Sora]

whenever I see people use python water changers strait from the tap, im always confused because its not pre conditioned before it goes into the tank. so what im wondering is, can you add strait tap water to your tank and use water conditioner after? is the actual chlorine harming the fish or is how it reacts and makes ammonia if its left in the tank harming the fish?

[Lennie]

Idk how it exactly harms fish, but it kills bacteria. That is a problematic part, may cause a cycle crash.

However, I've done it before and I've never faced any issues. I just dose the whole tank amount of prime, mix it all around, and fill the tank afterwards. That's the key I guess

Edited March 27 by Lennie

[RockMongler]

The chlorine can cause chemical burns to the fish's gills.  It does the same thing as inhaling chlorine gas in air breathing animals.  If you drain water, add dechlorinator, then add the water directly from the tap, the dechlorinator will work quickly to bind up the chlorine faster than it can hurt the organisms in your tank.

[jwcarlson]

I change 90% of water daily in my discus tank.  I dump the Prime in then walk to the basement to switch valves and set up my pump.  So I just pump the unconditioned water in and the Prime neutralizes everything (edit to add by "everything" I mean chlorine and chloramine) "instantly".  I would not put the new water in and then add the conditioner.  Put the conditioner in first, it will not hurt anything.  I've had a situation where a heater died and I didn't have pre-heated water to refill the tank and the discus skated around on their sides for 45+ minutes in an inch or so of water + Prime.  Then I added more Prime and pumped it full once I had the temp where I wanted it by hauling near boiling water down the steps a bunch. 😄

I also treat the whole volume of water when I change any appreciable amount of water more than a top off.  Meaning yesterday when I changed about 60% of water in my 37 gallon community tank, I treat all 37 gallons not *just* the water I changed out.  

Edited March 27 by jwcarlson

[AllFishNoBrakes]

I use a Python to refill my larger tanks. I just dose a whole tank amount of Complete as the water starts and I’ve never had an issue. 
 

I’ve also heard of others doing the conditioner after the water is full with no problems. I just have my habit of dosing as the water starts so I’m sticking with my habit. 

[The endler guy]

On 3/27/2023 at 12:42 PM, Sora said:

whenever I see people use python water changers strait from the tap, im always confused because its not pre conditioned before it goes into the tank. so what im wondering is, can you add strait tap water to your tank and use water conditioner after? is the actual chlorine harming the fish or is how it reacts and makes ammonia if its left in the tank harming the fish?

I have well water, so no chlorine for me!

also the pert of chlorine that harms the fish is, well chlorine 

as for why it harms, the boring chemical burns (not the fun nitrite poisoning where the blood rusts!)

Edited March 29 by The endler guy
Auto correct doesn’t even work anymore

[Pepere]

On 3/27/2023 at 1:23 PM, AllFishNoBrakes said:

use a Python to refill my larger tanks. I just dose a whole tank amount of Complete as the water starts and I’ve never had an issue

Just as a note… this works fine with Prime and Fritz complete….  I ran out of Fritz Complete once and local Petco only had Aqueon conditioner.

 

I did a deep water change and dows for the tank and started filling it up.

 

All of my Cories showed instantaneous deep distress.  I immediately transferred them to another 1 died within minutes, lingered for a day and died horrible deaths, 2 others took several weeks before resuming normal behavior.

Not all dechlorinators  react the same.

 

 

[AllFishNoBrakes]

On 3/27/2023 at 12:49 PM, Pepere said:

Just as a note… this works fine with Prime and Fritz complete….  I ran out of Fritz Complete once and local Petco only had Aqueon conditioner.

Understood. I used prime for the first 1.5 years of my hobby, and then the price up and I switched to Complete for the last 1.5 years. The 16oz bottle of Complete is very reasonable from ACO, or I can get either with my store credit. It’s worth the investment for me 

[Biotope Biologist]

Depends a ton on your municipality and how much chlorine they put in the water. Here in most parts of Western Washington chlorine tests rarely come back over 1.0mg/l. I’ve even seen it in as low as unregistered. Although I’m just using paper test strips, not the most accurate things in the world. Sublethal levels for aquarium fish is between 1.5-2.5 mg/l.

 

anyway chlorine stresses occur over a long span like 72+ hours. They do damage fish gills and pretty much any living tissue, it’s quite the nasty chemical. But dechlorinators are pretty efficient at locking it up.
 

Also interestingly certain European countries are moving away from chlorine as we are finding that the sun with enough exposure time is very efficient at killing waterborne pathogens. It’s fun learning about different water purification techniques!