Disaster occurring daily!

[mangelo]

I have a 165gal FOWLR that has been up for years. I started it bare bottom. I've kept ammonia at 0, nitrites at 0, nitrates 5 or less. Haven't lost a fish in a very long time.

I decided to go back to live sand. Days after adding the sand, I did a water change. Params were the same. Nitrate at 5.

After about 2 weeks, I started losing a fish daily. Fish that were eating the day before, were gone the next morning. All params are the same except for nitrates!!! It is off the chart!! I did a 40% water change. Nitrates didn't budge! I did another 40% water change. Nothing.

I have lost a fish daily for the last 2 weeks. I don't get this at all. Believe it or not, my Powder Brown is still alive as is my Foxface and Sleeper Goby.

I am preparing to do yet another 40% water change, but I have a feeling it will be in vain.

Any thoughts would be greatly appreciated!!

[ThRoewer]

This sounds like you introduced a highly virulent pathogen like Amyloodinium.

Where did the live sand come from?

[mangelo]

I added 3 - 40lbs boxes / bags. I got this from chewy.com

Nature's Ocean Bio-Activ Live Aragonite Saltwater Aquarium Sand, Natural White

[ThRoewer]

Is that harvested from the ocean, or worse from a site near a river mouth?
If yes, then there is a realistic chance that a Amyloodinium cyst or another equally deadly infection hitched a ride.
Did you observe the fish scratch, breathe accelerated, or behave in any other way suspiciously before they died. How do the survivors act?

[mangelo]

I did notice some scratching from some, but not all. The survivors, one of which is a Powder Brown Tang, is actually just fine which is an absolute miracle!

[ThRoewer]

Aside from the sand, did you add anything else in the last 3 months?

The powder brown tang may have immunity to whatever killed the other fish.

[mangelo]

I added other fish from a local fish store that uses copper. I've known the owner for a long time and trust what she says.

Other than that, I have not added anything else.

If I choose to get the remaining fish out and let the tank set empty for 2 months, do I have to get my eel out as well?

[ThRoewer]

How long ago did you add those fish and did any of them survive or where they among the initial victims?
Most stores do not use copper at the full therapeutic level and even if they did it wouldn't necessarily guarantee that the fish are clear of Amyloodinium since some strains of this parasite can survive 1.5 times the therapeutic dose.
Also, a copper treatment below the lethal level for Amyloodinium may just suppress it for up to 6 weeks. And a fish may have partial immunity to it and be symptom free while still carrying it.
For all those reasons you should always quarantine (isolate) new fish for at least 2 months and observe them closely.

[ThRoewer]

Originally Posted by mangelo

...
If I choose to get the remaining fish out and let the tank set empty for 2 months, do I have to get my eel out as well?

It's a fish, so yes, it needs to go out as well.

[mangelo]

I am not doubting that there may be something in my tank. I always quarantine, but didn't this time. All of the new fish parished first.

Why the extremely high nitrates? The color of the test is so dark red its almost purple!! Could that not be killing my fish?

[ThRoewer]

Did you find all fish that died? One decomposing somewhere might be enough to spike the nitrates.
High nitrates on their own are not immediately and directly dangerous to fish. Ammonia would be the thing to be concerned about.

The new, not quarantined fish dying first and the one-by-one deaths of the established stock is pretty consistent with Velvet/Amyloodinium.