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This happened a few weeks ago but I thought I would post my problems here so that others could avoid it, or finally solve a similar mystery that may have happend to them:

I have a 75 FW tank that has been setup for over 2 months. I have had all my community fish in there for around 2 months. I recently decided to order some Discus, SAE, and some more cardinals for the tank. The shipment came on Thursday. The fish were placed into the tank and each bag was acclimated slowly.

The Discus were stressed and appeared to be loosing their slime coat so I added more Prime to the tank and also some aquarium salt. The salt I added was a fairly small amount compared to a normal treatment (1/4cup vs 1 cup). I used less due to plants in the tank but I felt the need to help the slimecoat on the Discus mildly. A few cardinals died that night but I chaulked it up to stresses of shipping. I decided to do a 33% water change that night to be safe.

However, the next day 16 of my 20 cardinal tetras were dead as was 1 Discus! They remaining 4 were at the top breathing heavy like they had ammonia poisoning, but the test kits showed 0 for ammonia and nitrite. I turned down my CO2 which was not pumping out that many bubbles per second to begin with. I removed the 4 remaining cardinals and put them in a bucket with fresh tap water (Primed) and immediately they looked better. This led me to believe that it was definately a chemical poisoning the fish in my tank, not a disease and the cardinals are just the most sensitive.

Could the salt cause some reverse osmotic conditions in a particular plant to release something toxic to fish? I did not own any exotic plants just your standard LFS types. The only other thing I could think of was that 2 days prior to my receiving the shipment of Discus I decided to clean my tank and move some plants around. I had an anubis nana that was covered in spot algae and some hair algae. I removed it from the tank and one end smelled rotten. I cut that end and dipped the leaves in a mixture of H202 and water for ~1hr. I rinsed it with water and placed it back in my tank. Since the incedent, I have removed this plant...just in case it was leeching something into the water.

I did a 33% water change (stated above) after seeing the cardinals breathing up top, and another 50% change the next day after finding most of them dead. The 4 I moved to the bucket are all doing fine. The other fish were starting to look stressed, hiding, ocassionally breathing at the top...etc. They would look normal for awhile, eat and then go hide. I decided to break down the 75g tank and moved everything to freshly setup 20g tank. I moved a few plants over, and one of my filters.

24hrs later they all look normal. Colors are bright, they are swimming normally and even the 2 surviving Discus are starting to eat. WTF happend to my 75g?!:

1.) H2O2 plant reaction w/ or without help from aquarium salt (reverse osmosis).
2.) Some fungus remaining on driftwood became toxic...at just the right time...not buying that, but throwing out the driftwood since fungus has not settle even after 2 months.
3.) Something toxic on the outside of the fish bags that were placed in the tank to acclimate. I never even thought of rinsing off the outside the bags before putting them in my tank. Or from the newspapers they were packed in, heat packs...etc.


Now here is what I discovered after much searching. The culprit was most likely the cold winter climate. During warmer times of the year, we don't have to temper the cold water with as much hot water before adding it to the tank. In the winter the cold line is much colder so we have to use more warm water to get our tap to the temperature we want. With a Discus tank, I was getting my tap to 84F.

Hot water contains less oxygen than cold water. In a typical water heater, much of the oxygen in water is depleted because the heater is ~ 120F (avg. thermostat temp setting of water heaters). When we put this oxygen-depleted water into our tanks, it takes a while for it to re-acquire its dissolved oxygen. This is why you may find fish gasping right after a water change in the winter. When I changed 33-50% of my tank water, certain fish felt it and responded right away (cardinals). Thinking this was due to a chemical toxin, I panicked and changed even more water. This turned out, made the problem worse. Now I fill a 20g trash can with cold water the day before and heat it to my tank temperature prior to adding it.

Just thought I would put this out, so that no one else runs into this problem during the winter months. Here was my old setup:

http://www.aquariumforum.com/f15/20-75g-transplant-1244.html
 

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Pleco n bn breeder n BOSS
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Have you been adding salt all this time with water changes? If so it could have been a build up of salt causing a lot of discomfort to the fish. Salt isn't really needed in a tank unless you are treating for a specific decease that calls for it. Cardinals and Discus come from very soft water and there is very little salt content in the water.
 

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No, salt was just used once as a mild treatment for new fish. It is supposed to help with breathing. I typically NEVER use salt or any other medication in a planted tank if I can help it.

Salt was specifically mentioned by various Discus owners as a mild "cure-all" for stress, and injury on new Discus. With every water change I add a small known amount of acid/akaline buffer and equilibrium. I keep my tank at kH3 and GH3.
 

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I like your theory, and that is absolutely true about warmer water making O2 less available to the fish.

Also, my husband insists that I not draw water from the tap that will be used for drinking (i.e. filling a kettle, filling a pot to make soup or boil pasta) with HOT water. He says that hot water out of the tap leaches chemicals from the pipes that ought not to be consumed, and water that we want to drink hot should be drawn from the cold tap and then heated. We don't have lead pipes but he says the solder for copper pipe and possibly hot water through PVC is toxic.

Anything to that?

I will have to go through my journals and see if there is anything to indicate the problems I may have had with my tanks occurred during the winter. In the summer the water out of the cold tap is exactly the right temperature for my tanks - no hot needed at all.

This is an interesting situation - thanks for posting!
 

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Pleco n bn breeder n BOSS
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The only way its going to leach into the water is if the water is sitting in the pipes. Let the hot water run for a couple of minutes and there shouldn't be any harmful metals in it. I use warm water on all my water changes. Just during the winter months there is so much gaseous exchange that you need to let the water sit at least 24 hours if you are planning a big water change.
 

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Well you have to remember you have pipes (often copper) from your main to your water heater, and depending on the size and type of water heater you have, you still have that standing water enter your heater and water in your heater is standing also. I have learned to just minimize my use of hot water. I just fill a 20g trashcan of cold water the night before, let it get to room temp (~72F), then heat it up a bit more the day I add it.
 

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to save time why dont you add warm water to the tank
it will reduce the heater having to run straight away and it will also reduce the risk of stress to the fish

as for oxygen do you have air hoses in the tank?
 

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to save time why dont you add warm water to the tank
it will reduce the heater having to run straight away and it will also reduce the risk of stress to the fish

as for oxygen do you have air hoses in the tank?
WTF?

That is exactly the problem! I am saying that it is bad to use warm water from the tap (sink) because it does not have any oxygen in it. I am talking about the water heater in your house not in your tank.;)

I do not use any airstones in my tank. They don't actually add any dissolved O2 to the tank. The best way to add oxygen is to provide surface tension via strong filters near the top. This I have, however I limit the amount of tension because I am also pumping in CO2 for my plants. If the plants are happy they will produce enough O2 for your tank on a daily basis.
 
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