Neon Tetras suddenly died

[Mama Bookworm]

I have a 90l planted tank (no CO2) which is about 9 months old. Everything was going really well. I had neon tetras, shrimp and assassin snails. Water quality tests perfect, but over the last 3 days, 16 of my 18 tetras have died. There is no visible signs of disease and I hadn't noticed any change in their behaviour or eating habits. The shrimp seem fine, as do the assassin snails and pest snails. I tend to do a 50% water change every week, but since finding the first dead tetra I've done smaller water changes every day. I give the substrate a good clean every water change . My plants are all growing well and I have to trim them back most weeks. Any help / advice on what has happened will be very gratefully received.

 

[JeffK]

What are your water parameters and how do you test them?

Do the neons show any signs before dying?

 

[john6]

Have you used any air fresheners, sprays, candles, Laundry near the tank, anything at all of that description?

 

[Mama Bookworm]

I use test strips.
Ammonia, Nitrite and Nitrates all 0 (all the lowest on the test strip),
pH 6.8
Chlorine 0
I live in a very hard water area.
The Neons didn't display any unusual behaviour. The only thing slightly out of the ordinary, thinking about it now, was 5 or 6 were swimming near the back of the tank, with the rest shoaling together near the front. However, when I fed them they joined the others.
I have kept fish for years, but this is my first aquascaped tank.

 

[Mama Bookworm]

Have you used any air fresheners, sprays, candles, Laundry near the tank, anything at all of that description?

No, nothing at all. I'm completely flumoxed.

 

[john6]

although most hobby grade liquid test kits are not very good, test strips are as good as sticking your finger in the tank, licking it, and getting the test results, it will be more accurate.

 

[john6]

Maybe an ammonia spike perpetuated by death after death of the fish?

 

[Hufsa]

Strange.. you have very hard water but ph is 6.8, that doesnt entirely add up in my mind 🤔
Do you treat the water to get this ph?

I dont have any personal experience with strips, but they are said to be much more inaccurate than liquid tests. Maybe it would be worth testing with that. 16 in 3 days is pretty significant

 

[Mama Bookworm]

although most hobby grade liquid test kits are not very good, test strips are as good as sticking your finger in the tank, licking it, and getting the test results, it will be more accurate.

I have just tested again using liquid (API) test kit and got exactly the same results.
I don't think ammonia is a problem as I knew how many fish were in the tank and I have 'fished' out all the bodies.

 

[Mama Bookworm]

Strange.. you have very hard water but ph is 6.8, that doesnt entirely add up in my mind 🤔
Do you treat the water to get this ph?

I dont have any personal experience with strips, but they are said to be much more inaccurate than liquid tests. Maybe it would be worth testing with that. 16 in 3 days is pretty significant

I have a water softener attached to my water system.

 

[G H Nelson]

Hi
What is the Gh/Kh on the test strip?
Where are you drawing the aquarium water from?

 

[bazz]

I have a water softener attached to my water system.

What, a salt based softener on your incoming water supply from which you do your tank maintenance?