Lost all but bichirs overnight

[duanes]

When you came home, was there a thin white film on the surface of the water? Beside the water being being cloudy?
If so, the agitation provided by the air pump (when working) allowed constant air exchange).
But when the air pump failed, if there was thin film (that quickly blocks air exchange, it can suffocate the fish that need high oxygen levels like clowns, and cichlids very quickly) In a well stocked tank where filters are seldom cleaned , and water changes are not frequent enough to remove organics like nitrate, a few hours of dropping oxygen levels can be lethal.
I mention this because you said the filter was cleaned 2 months ago. To me that length of time between cleanings makes the filter a negative influence on air exchange. And with that stocking level, would to me indicate that 40% water changes should be done at least twice per week on a normal basis.
The air water interface is at least as important factor in tank health as vacuuming from the substrate, this is where many oils, proteins and toxins build up, so in a well stocked tank surface skimming is paramount. (Another reason I find canister filters with intakes well below the surface inadequate)
If a can was my only filtration option, I'd add one of the commercially available surface skimmers.

 

[squint]

That air pump isnt putting o2 in the water by being on. Its allowing some small amount of surface aggitation to allow gas exchange. The air pump being off didnt cause this.
How long has the tank been set up?
Does the output from your canister disturb the surface at all? It should if not.
Sorry for the questions I know this is hard, but that air pump off for that short time imho is not/could not be the cause.

Oxygen transfer from bubble aeration is twice that of the surface transfer due to the surface aeration of bubbles breaking on the surface.

Bubble aeration is also vastly superior to surface aeration (which apparently does next to nothing).

If the tank had enough oxygen demand then the air pump could have been all that was keeping the fish alive.

 

[RD.]

A lot of unknowns, but I will add this. Years ago there was a local gal who kept a heavily stocked African cichlid tank, not crazy overstocked, but IMO overstocked. It was a 90 gallon filtered by an XP3, she also ran a large air wand for extra surface agitation. She went to bed one night at 11 PM, all was well. At 8 AM the next morning all of her fish were dead. The only thing that had changed was the air pump running that large air wand, had stopped sometime after she went to bed.

A classic case of 02 depletion, which it sounds like is what quite possibly happened to the OP's tank.

 

[skjl47]

Should I cycle the whole tank again?

hello; hard to say for sure. Water changes alone should not kill off the beneficial bacteria. A couple of possible exceptions to consider. One is how long the bb have been without their nutrients (in other words have enough been starved out?). Without fish or some other source of ammonia the population of bb will decline. One positive in your situation is there is some ammonia released when decay occurs.
Another is what sort of chemicals the tap water had when you did the massive rinse out and water replacement. I get you turned off the filters during so the bb in them should still be OK. However the bb form colonies on many surfaces in the tank and all that tap water with chlorine or chloramine may have wiped out he bb on the exposed surfaces.

If you do not let too much time elapse then a cycle from scratch should not be needed as you should have some bb colonies left alive. You also mention another tank(s) so could rob bb from that.

lost 3 clown loaches, 2.5 inches, 2 acara, 3 inches, 1 pike cichlid 4 inches, 1 fire eel 12 inches.

Surviving,
1 ornate bichir, 14 inches.
1 speckled bichir, 10 inches.

Hello; Not a huge population for a 120 on the face of it. Hard to say if the stocking level alone was enough. So much would depend on factors such as over feeding or just that one of the larger fish died first. To me a bubbler has always been critical in a well stocked tank, especially back when I overfed.

That air pump isnt putting o2 in the water by being on. Its allowing some small amount of surface aggitation to allow gas exchange. The air pump being off didnt cause this.

hello; I must disagree with this conclusion. With the benefit of several years of running tanks on the edge many decades ago one thing I did learn was a god bubbler made the difference. I overstocked (well at least had heavy stocking) and tended to feed too much as well. Back in the early days of power filters they failed often and many times a god air pump was all that kept my tanks going.

In a well stocked tank where filters are seldom cleaned , and water changes are not frequent enough to remove organics like nitrate, a few hours of dropping oxygen levels can be lethal.

Hello; This.

filter was cleaned 2 months ago. To me that length of time between cleanings makes the filter a negative influence on air exchange. And with that stocking level, would to me indicate that 40% water changes should be done at least twice per week on a normal basis.

Hello; This.

Bubble aeration is also vastly superior to surface aeration (which apparently does next to nothing).

If the tank had enough oxygen demand then the air pump could have been all that was keeping the fish alive.

Hello; This also.

 

[Belly up]

I recently had something like this happen to my 75. I had fed my BN plecos green beans without noticing my hob had quit. A few hours later I came in to discover a very cloudy tank and fish gasping at the top of the tank. I immediately did a water change and corrected the filter mishap. I lost a prized (to me) arulius barb and almost lost the whole tank of fish. I have since moved some fish to lessen the bioload and added a sponge filter to that tank in case the hob ever quits again.

Frustrating isn`t it? I would be truly bummed had I lost more of those fish and feel for you.

 

[twentyleagues]

If the air pump was the only means of surface aggitation and there for the only means of adding o2 to the tank then yeah that could be it. I had a 75g way overstocked when I first started posting here. I knew I was upgrading so I didnt care plus my maintance was where it needed to be. I didnt have an air pump on that till about 3-4 months before I upgraded. The only reason I added it was to get some more water movement to an area that was collecting debris, I didnt want to add a power head. Both of my canister filters output were above the water level, I also did filter maintenance on them monthly, one on or near the 15th the other the 30th. Never had an issue. With proper surface aggitation you do not need an air pump. Power went out for about 18 hrs during the summer right before I moved everything to the 180 didnt loose anyone.
I'm just saying, to blame it on that alone is not getting to the root cause. Could a lot of mismanagement have lead to this.....? Maybe? We dont have enough info really. A lot of fish have gone in and apparently out of that tank in a very short time if you go check the op history. Could have been a bug. Could still be.
Also I agree with what duanes said about surface skimming on canisters too. Very effective way of "hot rodding" a canister. I made a diy one out of pvc for around $6.

 

[RD.]

In the OP's scenario, with such a short duration of time, I can only imagine two different possibilities - 02 depletion, or a toxin being introduced to the tank.

I don't run air stones either .......

 

[Coryloach]

In the OP's scenario, with such a short duration of time, I can only imagine two different possibilities - 02 depletion, or a toxin being introduced to the tank.

Exactly my thoughts too.

Oxygen depletion is not that rare. It is rare to lose all fish at once but unexplained random deaths due to oxygen dipping below fish tolerance level is not that uncommon, only that people assume every other possibility but oxygen...

Oxygen is really difficult to dissolve in water...To quote a previous post...

"Oxygen is absorbed in water by direct diffusion and by surface-water agitation. Solubility of oxygen in water is so small and by diffusion process alone in still water, it was calculated that it would take 6 years for oxygen to diffuse from surface to a depth of 6 meters in quiet water. Absorption of water is very minor, that almost all the oxygen enrichment of natural waters takes place by agitation of water."

http://www.fao.org/tempref/FI/CDrom/aquaculture/a0845t/volume1/docrep/field/003/ac183e/AC183E04.htm

 

[The Morning]

I would lean towards oxygen depletion as well. When I upgraded from my 55 to a 150 I added a second Eheim canister and removed 2 HOB thinking this would be more than adequate. Unfortunately what I forgot was to aim the spray bar towards the surface on either filter. With the HOB packaged away there was no longer adequate surface agitation. I lost most of my bigger cichlids in less than 24 hours. Lesson learned. I know have one spray bar aimed up towards surface and added wand type air stone.
Sorry for your lost fish.

 

[twentyleagues]

So everyone is kind of in agreement that there needs to be in some way or another more surface aggitation to help increase oxygenation. Either the reason or not better airation is a must, id do both get the outputs above water line and get the air pump working again.
Hope this is the only reason and not a toxin or a bug. You might want to run some fresh carbon in the canisters for a few days too just in case a toxin got in the tank.