Ember Tetras dying

Good day

Last Sunday (27 September) we bought 8 Ember Tetras. I set them up in a quarantine tank, acclimated them and then let them get on with being quarantined. I added a low dose of aquarium salt (1 tablespoon per 3 gallons) to the tank to help combat any illness or parasites they may have. I did have an issue with the heater being set too low, but I systematically bought the tank up to the match the temperature of the main tank (26.5 - 27 degrees).

Tuesday we lost one fish that had been swimming a little bit separate from the school. Yesterday I bought a piece of rock with an overhang, thinking they may not have enough hiding place(they were always flitting away and hiding under the heater or under the foam on the intake of the HOB).
This morning we lost another fish, and this afternoon yet another.

I cannot see anything specific wrong with the fish, they aren't bloated or discoloured, no ich-like spots or velvet-like textures. Their eyes are even still clear.

I did a minor water change earlier in the week, and did a 50% water change now after finding the last fish. All new water is treated with Seachem Prime before being added.

I did a test on the old tank water, pics included below.

Does anyone have any other ideas?

 

I am assuming that your quarantine tank has a cycled biofilter and that you are only feeding them sparingly. I know Prime will sort out ammonia, nitrites and nitrates. But without a cycled filter for a week, would the initial amount of prime added still be sufficient? If the biofilter is not mature, would the ammonia spike not slowely increase within a weeks time regardless of the Prime added originally? I dont know which is why I am asking.

How is their feeding response? Does it seem like they are eager to feed or do they seem lethargic? From what you are saying they seem skittish? I would try to distinguish whether their behaviour stems from compromised health status (which seems like you have ruled out) or the environment you are providing. I would try to cover all the tank panes except for the front pane with some dark material to make them feel more secure. Stress is a big driver and could be a trigger for other underlying conditions

In the past I have had fish hang out by the heater as well (due to lack of hiding space) and had several deaths. My tank flow was also minimal so I suspected hat steep temperature gradients developed within the tank. I was also at a loss and started wondering whether the fish stayed in close proximity to the heater for too long and whether this could have caused some physical damage. Still not sure.

Water parameters seem to be in check. Do you perhaps know if your water parameters differ by much from the LFS that you bought them from? I realise that within a week they should have adapted to your water conditions. Just wondering whether the acclimation time that you provided initially from the store bought water to your tap water was long enough. This can cause shock to their system and symptoms can potentially be delayed. The added salt could also have increased the TDS by some margin depending on the original water quality.

Did you dose any medication into the tank aside from the salt? If so, is it possible that you overdosed or that the medication has expired.

Other than that, all I can think of is that some harmful substance found its way into your tank. Maybe a residual contaminant left over after washing / cleaning the tank? Maybe the rock work that you added?

Just some subjective inputs to consider