Tetras slowly dying off

[Trog08]

Hey Fish peeps

I've recently made the potential mistake of setting up a small 65 liter (17 gallons) aquarium for my aunt, stocked with 10 cardinal and 10 ember tetras (I know this may be slightly over stocked, she bought the fish without me). For the past week she's been having some die-off of the tetras. One or two a day and now shes down to five cardinals and 8 embers.

The tanks been running for a month and a bit now, cycled using existing media from one of my larger aquariums. I tested the water on Saturday and did a 25% water change after. I used the API master test kit except for GH/KH where I only had tetra test strips on hand. The test numbers were:

PH: 6.8ppm
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 10ppm
KH: Between 4 and 8 (only had test strips to check)
GH: Between 4 and 8 (only had test strips to check)
Temp: 24 degrees Celsius (75.2 Fahrenheit)

I'm really sorry I don't have a picture to attach of the tank. But it has Fluval stratum substrate, multiple plants (Anubias, java fern, and crypts). There's also a piece of spider wood and dragon stone. The filters are an Aquaone HOB and a sponge filter.

Any ideas what could be the culprit of the dying fish?

Thanks All!!!!

[Theplatymaster]

@Trog08 are there any symptoms before the deaths? or do the seemingly healthy fish just die?

pictures of the fish and tank would be very useful.

how big are these tetras? is an intake sponge being used?

Edited February 20, 2023 by Theplatymaster

[ScottieB]

If I had to guess given the info it was probably an ammonia spike. A lot of bioload even with some established media likely tipped the scale causing the demise of the newly introduced fish. Totally just guessing by the way.

Also consider the source of the fish, maybe they were already doomed and no matter what. You could consider starting the Coop med trio as a prophylactic measure. assuming you saw no other symptoms. 

[John Henry]

Could auntie have done a water change or top off on her own without using a de-chlor agent?

[Trog08]

@Theplatymaster I’ll try and get photos next time I’m there. They’re the standard cardinal size and ember sizes and are behaving completely normal, swimming around the tank leisurely and no signs of stress. There is an intake sponge on the filter. 

@ScottieB I’m is Australia and we can’t get the med trio which sucks immensely! I was concerned about an ammonia spike as well but I couldn’t see any symptoms of it. There are also some rams horns in there that hitched a ride on the media and have since grown. They don’t seem to be affected by whatever is happening so I’d ruled out the ammonia spike. I did get her to dose double strength Prime after the first couple of fish died in case of ammonia before I could do the water change and test.

@John Henry at the moment I’m doing all the water changes, she’s too scared of even getting within 3ft of the tank because she doesn’t want to scare the fish.