Hello There,

I builded a planted tank four months ago, 80 liters heavily planted, EI dosing and CO2 inject. I waited 3 months to add the fishes.

Since Im doing EI dosing, I do a 50% water change on saturdays. As I was afraid the fish would suffer, After I get out the water, I refill it very slowly. Taking about two hours to get the tank full.

But after every water change my Cardinal Tetras start acting weird and next morning they are dead.

I had 30 cardinal tetra, all of them died along 3 weeks. Right after my water changes.

What am I doing wrong? is EI dosing incompatible with more delicated fishes?

Thank u all.

I had that issue once,turned out my well was contaminated with a grain fumigant.The solution for me was a carbon filter on the water line until I got a new water source.

PH, KH and GH readings of both the tank water and also the change water? If they don’t match you need to make them match. Might be PH shock, might be osmotic shock, might be both.

Also is your source water chlorinated and are you using prime etc right before adding water to tank if it is?

Buy a TDS meter (they're cheap) and keep it calibrated. This will tell you if you are risking osmotic shock (I no longer believe in the concept of pH shock). I would suggest never allowing more than a 50 ppm or 10% change in TDS, whichever is greater, in a 24-hour period.

Of course, all of the other 'nasties' , mentioned above, may be in your source water that can act as poisons, as well.

Are you dechlinating the water before you add it to the tank? Chlorine is often added to tap water to kill bacteria, viruses and parasites. If you don't pretreat the tap water the chlorine could kill your fish. Seachem Prime water conditioner is one such product.

What does "acting weird" entail?
What is the temperature of your tank vs. the water entering the tank?
Are there lots of air bubbles on the sides of the glass inside the tank after your water change or even on the fish themselves?
Is the water being put into the tank above the tank so it splashes on the surface or below the water line at any point?

I know you said you waited 3 months to add fish, but were you adding ammonia in that time? Is the tank cycled? What are your parameters (ammonia, nitrites, nitrates) before a water change? Also, are you monitoring the temperature of the new water before you add it in? If it's a few degrees off you can shock your fish.

I think temperature is unlikely to be an issue if it's taking 2 hours to fill the tank.

Cold water from the tap added to a warm tank can increase the risk of gas bubble disease.

Temperature is extremely important.
Ridiculous amateur mistake 2 days ago cost me a school of rasboras due to temperature difference.
I would consider looking at the temperature carefully

I agree that temperature is very important when adding water rapidly to a tank! But if it's being dripped in over the course of a couple of hours, I don't think any reasonable range of temperature will have much impact - it will all be room temp by the end anyway, and added slowly enough that the tank's heater can accommodate it.

One thing you can do on smaller tanks, maybe 20 Gallons or less is fill up some one gallon water jugs and leave them somewhere right after a water change. Then on water change day use those instead of the tap or use the jugs and some tap. This way you have some degassed water and the temp change won't be as drastic.

Are you running your co2 while you do water changes? I experienced something similar a few month back when I nearly killed my ghost knife fish. I did a 50 % water change also, my ph meter was set at 6.4 for weeks with all fish fine. Then when I changed the water I forgot to turn off the co2 while it still injected at half the water volume, extremly bad in that case my fish nearly died but luckily was able to save it. Point is some amounts of co2 seem to be fine in specific amount of water volume but not in others.

It could be any one, or a combination of the following...

- tank cycling
- Source water quality
- Chlorine/chloramine and/or how and how much conditioner is used.
- temperature
- dissolved CO2
- You may need to back off on EI and reduce the volume of the partial water changes