Help!! White clouds keep dying

I have a mystery that I cannot work out. I have a tank of death
My tank cycled the paramaters are all good. I test with API kit, I have taken the water up to my fish shop three times to verify the results and also have dipstick test. I am testing the water at least once a day and have not identified any spikes.

I had an axolotl who was tiny when I got her, (3cm) and missing two legs. She was the only thing in the tank and seemed to be improving but developed a fungus. I was trying to treat the fungus and she was looking heaps better and then all of a sudden she got really ill and died in one morning. At this stage she was still only about 5cm so super hard to treat.

Despite my water always reading ok, I was worried maybe there was a problem so I decided to get some White cloud minnows. Well, that hasn't worked form me as I cannot keep them alive more than a few days at best. I suspected that maybe the stock was the problem in our area so i got one comet goldfish. Her name is Lena and she has had no problems.
I thought that I was ok, as I had five minnow and they all survived 24 hours and seemed happy so I went and picked up some more.
Now they have all died again and I think I only have two left. I am at a complete loss. My fish shop, which is a specialist fish shop, is also at a loss.
Please help, further details below.


Tank
What is the water volume of the tank? 123L
How long has the tank been running? 6 months
Does it have a filter? Yes
Does it have a heater? No, Chiller
What is the water temperature? 19 degrees celcius
What is the entire stocking of this tank? (Please list all fish and inverts.) 1 comet goldfish (small) White clouds up 12 but in various numbers

Maintenance
How often do you change the water? once per week
How much of the water do you change? 25-50%
What do you use to treat your water? Api Water conditioner
Do you vacuum the substrate or just the water? vacuum the sand substrate and rake

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes, cycled and housed an axolotl, the axolotl was small and missing limbs when I got it and it eventually died.
What do you use to test the water? Api master kit, Testing at local fish shop and test strips
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 0-5
pH: 7.2

Feeding
How often do you feed your fish? 1-2 times a day (if feeding twice smaller amounts, due to bringing fish in ect)
How much do you feed your fish? only as much as can be eaten in a few minutes
What brand of food do you feed your fish? Not sure
Do you feed frozen or freeze-dried foods? Flakes the coldfish, insect pellets for minnows

Illness & Symptoms
How long have you had this fish? Gold fish has been in the tank for over a week. Minnows are currently not lasting more than a few days at best
How long ago did you first notice these symptoms? -
In a few words, can you explain the symptoms? White clouds seem to stay at the top of the tank. After a while they start to be pushed around the tank with the waterflow and just float around the tank until they die, can float upright, sideways or upside down at any time, or they sit on the bottom of the tank and stay there until they die.
Have you started any treatment for the illness? No, it looks like they need more air but my filter drops over 5 cm into the water creating visible bubbles in the air. In desperation I put an air stone in yesterday just in case and ran it for a couple of hours but then had to turn it off overnight due to noise and it is in the bedroom. This morning most of the fish were dead. I have turned the air stone back on, one fish is floating around so probably on its way out the other two look ok.
Was your fish physically ill or injured upon purchase? not that I am aware of, but when i visted another pet shop they had lost most of their minnows from their shipment that morning. I am wondering if fish are getting too stressed during transportation (Australian summer, COVID delays)
How has its behavior and appearance changed, if at all?

Explain your emergency situation in detail. (

 

I think this has something to do with water Parameters and how they are not fit for white clouds. White clouds like cooler temps Also do you know what your KH and GH is. Are there any spots or blood burns on the fish that died.

 

I think this has something to do with water Parameters and how they are not fit for white clouds. White clouds are tropical and you need to keep them at 73-79 F. They are gonna need that type of tropical water. Also do you know what your KH and GH is. Are there any spots or blood burns on the fish that died.

this information is incorrect. White clouds are very happy at cooler temperatures.

This sounds like shock. 1. stop buying fish, wait. 2. how do you introduce your fish to your tank?

 

this information is incorrect. White clouds are very happy at cooler temperatures.

This sounds like shock. 1. stop buying fish, wait. 2. how do you introduce your fish to your tank?

Hm my bad. That's what I actually intially thought but I wanted to confirm myself. I did more research and I found an article (actually a couple) that said they enjoyed warmer temps. Thanks for the check i really appreciate that. My bad!

Hm my bad. That's what I actually intially thought but I wanted to confirm myself. I did more research and I found an article (actually a couple) that said they enjoyed warmer temps. Thanks for the check i really appreciate that. My bad!

Changed it

 

The water temperature for minnows is well within their parameters, they do not tolerate warmer temps but should tolerate cooler temps no problems.
GH is 150 KH is 80. I do not have any pH spikes and the gold fish is fine.

I've put an airstone in to see if it changes anything, but like I said the goldfish is fine and I have a 5 cm drop from my filer.

 

The water temperature for minnows is well within their parameters, they do not tolerate warmer temps but should tolerate cooler temps no problems.
GH is 150 KH is 80. I do not have any pH spikes and the gold fish is fine.

I've put an airstone in to see if it changes anything, but like I said the goldfish is fine and I have a 5 cm drop from my filer.

Could you please explain how you add fish to your tank? As it sounds like they are going into shock.

 

Also are you getting these fish from the same LFS.

 

Acclimatising by floating for 15 min then adding water from the tank every 10 min, three times then using a net to move them. All the minnows are from the same shop, when I tried to get some from a different shop all theirs were dead. What does it look like when the fish go into shock?

 

Acclimatising by floating for 15 min then adding water from the tank every 10 min, three times then using a net to move them. All the minnows are from the same shop, when I tried to get some from a different shop all theirs were dead. What does it look like when the fish go into shock?

Hmmm weird. That sounds fine. They are dying so fast which would normally mean shock or bad water. I dont think its the shop. Something in your tank is wrong but I dont know what.

 

An option would be to set up a small quarantine tank with an airstone and see how they behave in there with the daily water changes.