Long Time Fishkeeper With Newbie Problem

[modernhamlet]

Ok, I have had 75g and 40g mbuna tanks for 5+ years. Was a regular here for several years. I know about cycling. I preached cycling for years.

Yesterday, all of the fish in the 40g started coming to the surface, gasping for "breath". I tested and there had been a Nitrite spike to about 2.5ppm. This was completely out of the blue. This tank has had new fry as recently as last week.

I immediately performed a 50% water change (~1ppm), then another several hours later (.5ppm), and finally a third last night (>0ppm, <.5ppm). This morning they seemed a bit better, but this afternoon, they're back at the top and I'm about to lose 1 or 2 of them.

I've had no chlorine readings in all tests. Nitrate was initially at ~40ppm, now <5ppm.
I also did a water change in the 75g. The fish in that tank have shown no issues.

So the question is: Is this the effect of nitrite exposure that from which they'll eventually recover (as much as that's possible) or am I looking at something else entirely? If the latter... what is it?

 

[WILDER]

Increase aeration.

A high nitrite reading can cause gasping.
Are the gills red and inflamed.

How many fish and which type do you have in the 40 gallon.

 

[Miss Dib Dabs]

Have you checked filters for problems? That could cause a nitrite surge, leading to these problems. Also you say you've had fry in the tank recently. Is it possible there was a big amount of new fry or a number of deaths with the fry. Any of this could cause an issue.
Nitrate levels will drop with water changes. Add an airstone for oxygenation. My malawis love a high oxygen content in their tank, and my catfish.
P.

 

[modernhamlet]

Just back home. Lost three at same time as the water change. No new losses since then, but they're still at the top. I've made sure the filter outlet is bubbling the surface. Nitrites still low, just above 0.

We'll see how they're doing in the morning.

 

[Mikaila31]

Add anything to the tank at all? Even nonliving things? If its planted did you disturb the substrate alot lately?

I had a log that cause a major nitrite spike in my 55gal and took out most of the fish a year ago.

 

[modernhamlet]

Add anything to the tank at all? Even nonliving things? If its planted did you disturb the substrate alot lately?

I had a log that cause a major nitrite spike in my 55gal and took out most of the fish a year ago.

Nothing new in quite some time. Like I said, these are long established tanks.

It's just so frustrating to see them all up there struggling and have no idea what's going on. Even with the nitrite 2 days ago, they shouldn't STILL be at the surface, should they?

The standard options (and why i've counted them out):
chlorine spike (tested 0)
ammonia spike (tested 0)
nitrite spike (tested 0 for last 36 hours)
low O2 (4 water changes and filter outlet breaks surface)

The only other option I can think is that some unknown/toxic dissolved solids were introduced in the first water change.

What am I missing?

 

[WILDER]

I would add some fresh black carbon and filter floss to the filter.
Double dose the declorinator.