Something is killing all my fish - please help me figure out what

Something is killing all my fish.
Background.
Its a 150 Gallon African Cichlid tank.
Due to travel it didnt have a water change for a while. Nitrates were very high
This past Sunday I did a 50% water change. Within the next 2-3 days 11 fish died
I checked the all the numbers, Ammonia was about .25, Nitrites, between .25 and .5 and Nitrates were still showing up very high.
I did another water change yesterday. Same results right after. And fish keep on dying.
Something didnt make sense.
Today in the morning I did a 95% water change. And kept on testing the Nitrites of the water as the tank filled up. The Nitrites were staying at zero. I turned on my FX6 filter and 30 minutes later checked again, and the numbers were high again. I realized that the filter must be putting the bad chemicals into the water.
I did another 60% water change, and didnt turn on the filter and now my numbers are good.
Another fish died about two hours after I completed this, but it could have had issues from before I did the most recent water change.

Does it make sense that Nitrites at those levels could be killing the fish so fast? The way the fish died (if this helps), was they would be losing control slowly. They wouldnt be able to fight any current, they would stop moving and then if disturbed would move a bit and then stop again, this would continue until it died.

Any suggestions? I don't want to assume its nitrites causing the issue if it wasn't.

Also, I will be running the tank without the fx6 until sunday as that's the soonest the new filter media will arrive, but I will have on the back a new 110 filter.

Thank you

 

While yes nitrites are bad it makes me wonder if some kind of chemical got introduced to your tank as the deaths didn't start until after the water changes started back up.

Did you add anything to the tank?

Unfortunately it looks like the lack of water changes may have thrown your tank into another cycle which is super stressful on fish regardless. I'd do daily 50% water changes until your levels regulate to help the fish survive.

 

I would make sure to clean the filter media in the FX6 filter and either put it in the new 110 filter or turn that one back on... as that is where most of your microbiome is. If it doesn't have running water passing over it the colony will die (if it hasn't already) which would mean you would need to fully cycle your tank instead of hopefully just deal with a mini-cycle.

 

When you say no water change for a while. How long was that? When a tank has been neglected the fish slowly get used to the conditions they are living in. Sudden changes to fresh water can cause what you are experiencing. Smaller water changes done over a longer period of time may have prevented the deaths.

If your canister filter hadn't been cleaned in a long period of time it is possible that it is in pretty bad shape too. When you turned it back on after a water change it would have been pushing/pulling water through whatever build-up that had accumulated in there.

Are you seeing nitrites? The post starts out talking about nitrates but then goes on about nitrites.

 

I do have an existing 110 on back that I can keep. I was going to put a new one to make sure no nitrates were around.

There was nothing introduced to the tank extra. The fx6 when it starts up does a flush almost do maybe that's what got all the bad chemicals in


Thank you

When you say no water change for a while. How long was that? When a tank has been neglected the fish slowly get used to the conditions they are living in. Sudden changes to fresh water can cause what you are experiencing. Smaller water changes done over a longer period of time may have prevented the deaths.

It was 2 -3 weeks

If your canister filter hadn't been cleaned in a long period of time it is possible that it is in pretty bad shape too. When you turned it back on after a water change it would have been pushing/pulling water through whatever build-up that had accumulated in there.

The canister filter was probably last changed 6 months ago

Are you seeing nitrites? The post starts out talking about nitrates but then goes on about nitrites.

I am seeing both.

Thank you

 

It was 2 -3 weeks

That is good news and really shouldn't be long enough to cause what is happening. Do you know if your water company has been doing anything with the water lines in your area lately?

The canister filter was probably last changed 6 months ago

I am not sure I understand what you mean. Do you mean it was cleaned 6 months ago or do you change out the media? I've never used a canister filter so don't know if 6 months is too long between cleaning it or not. I would think it would get pretty bad though in that length of time.

I am seeing both.

Thank you

It is strange to be seeing any nitrites in an established tank but of course you already know that. How high were your nitrates before you started doing the water changes?

 

I would agree with mattgirl Given the type of fish you have 3 weeks would probably be enough for your tank to decline quite a bit. Your filter would have been dealing with a big bio load with the tank not being cleaned or having water changes in that time as well. Did you clean the filter as well?

Sounds like the shock of a big change in the water parameters.

 

The filter wasn't changed or cleaned during those months. When I took it out now it was filled with so dark filled with junk.

Before the change, I only measured the Nitrate and that was very high 160+

Hopefully, things are settling down now a bit and the water change was the cause, so they won't die any more.

Thank oyu

 

The filter wasn't changed or cleaned during those months. When I took it out now it was filled with so dark filled with junk.

Before the change, I only measured the Nitrate and that was very high 160+

Hopefully, things are settling down now a bit and the water change was the cause, so they won't die any more.

Thank oyu

I do hope you don't lose anymore of your fish. This is just one more warning to those that think they can get away with fewer water changes. All may be fine for a while but it can very well come down to this sad thing happening.

You may want to consider opening up and cleaning the canister more often. I feel sure it had something to do with the nitrates going sky high. Changing out so much water so quickly and dropping the nitrates to quickly could very well be the reason for the fish loss.