Desperately Need Advice Of Fish Dying After Water Change


The mystery of dying fish.
I have about 8 Yellow Labs. I did a water test in my 150Gal tank. Ammonia was slightly elevated Nitrites were elevated and nitrates were slightly elevated. My ph level was about 7.6 (which is my tap level).

I did about a 30% water change added prime. I checked my numbers, the ammonia was down to zero, as well as nitrites and nitrates. Ph was the same.

I left for about 3 hours came back and the tank was about 35% empty. I realized that I left the hose in the tank and it acted like a siphon. The fish though were swimming around ok. I refilled the tank. I added prime into the clean water as well as directly into the tank. If I needed to guess, I would guess that I did about 5 - 8 oz. (which is more then called for, but the tank was still having issues cycling). And I threw a splash of baking soda into the clean water source. I took the measurements and everything was the same. The water temp didnt budge more than a few degrees.

About two hours later I glanced at the tank and a few of the fish were dead. The others were bopping up near the surface before they eventually died as well.

So I am trying to figure out what killed the fish. Was it that I basically did two water changes one after the other (because of the siphon)? Or was it because I added to much prime?

It was not a Ph shock because of the ph level before and after were about 7.6

Thank you


I am suspecting it may very well be pH shock, due to the splash of baking soda you added. If the pH is steady at 7.6, why did you add it?

You might be able to save the rest with another water change to bring the pH back down to what they are use to, but I don't know if it will help. I hope it does.

Also...only dose enough prime to treat the new water to prevent using too much more prime in case it is a contributing factor. The large water changes will be enough to take care of the ammonia and nitrite for now.

86 ssinit

If ph remained the same it was probably something in your water source. Something in the source and the 2 water changes were just to much for the fish to handle.


^ ^ ^ That is possible too.


Excess Prime shouldn't hurt them.

You might check your town's water and sewer website to see if they did any maintenance, especially chemical flushes. They're rare but can devastate a tank if they coincide with a water change.

Also, a 150 gallon tank temp changing several degrees means some of the new water was much hotter or colder than their previous temp.


You used 5-8oz of Prime?

At 5 ounces, that is still slightly over 10x the normal dose for a 150 gallon tank.

What was baking soda for?


I don't know how sensitive those fish are, but even the most sensitive will handle a large water change fine when it's decreasing ammonia & nitrite. It's not common, but it is definitely possible to overdose prime, so I'm guessing that was a contributing factor.

How long has the tank been running and how long have you had the fish?


8 ounces can't be right... That would treat 2,370 gallons and is 15x the standard dose.

If that was the actual dose then the fish might have suffocated.



I've heard the Prime itself isn't toxic at high levels, but it reduces the dissolved oxygen in the water so they could have passed from that.

Fish gasping at the surface is a tell tale sign that they aren't getting enough oxygen.

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