Please help!!!!!!

[Guppy 1230]

Not bashing or attack. Critical advice can come across like an attack, but I assure you I have no ill will towards you at all.
What type filtration do you use?
In your first post you stated you “recently “ add these fish, going by that and what I see in the pictures it looks like this tank could not handle the bioload and you’ve had an ammonia spike.
You didn’t post any test results.
Did you test during this process?
Testing would have given you critical information as to what was happening to the water quality.
Also while it is true plants can help with ammonia and nitrate build up. What you have in this tank is very sparse. It would take the tank being very densely planted to see any real up take compared to what the fish can produce.

It has the under gravel filter, which I have never used before. I normally do sponge filters, I was worried about the ammonia when we added the killifish. It definitely cycled between putting the killifish in and two days ago. One of the balas had what almost looked like fin rot by its eye before they died.
I haven’t tested but probably should.
I moved the eel into a QT tank and had no luck, he passed sadly. My confusion is why was it only those fish? The spiny is fine and so are the killifish and the tetras.
I definitely wouldn’t consider it densely planted but I’d say about half of the back of the tank has plants.
Also I feel like it’s worth mentioning that one of the spineys has disappeared completely within he last two weeks. I’m not sure if he burrowed and couldn’t get out but there have been no remains or any evidence of him in the tank so I don’t know if maybe that added to the ammonia or not either.

 

[neutrino]

Sorry for these losses. You really need to get your water tested. That might answer any questions, if not then at least it may eliminate a strong and obvious possibility and you can move on from there. If it IS an ammonia (or nitrite) issue then you'll know (or can be advised) what to do. It does look and sound like you're dealing with an ammonia (and/or nitrite) problem, including the circumstances leading up to this. But not testing to confirm or discount it is flying blind at this point.

The eel looked pretty far gone, probably why moving to another tank didn't save him.

 

[Fishyboy🐟🐟🐟]

I recently switched my fish into a 40 breeder tank. We had a handful for bala sharks, two spiny eels, a tire track, and some black skirt tetras. We added in a pleco, cat fish, and some killifish. They’ve all been doing fine until this passed week, all the sharks died for no reason, same with the pleco and cat fish. Well now my tire track is not doing good, two days ago he was fine, yesterday morning I noticed his slime coat looked odd and last night it looked like boiled egg whites were coming off of him. This morning he looks even worse, I don’t think he’ll make it passed today to I want to treat whatever it is. I added salt to the water thinking maybe it is ich but I’ve never seen it come on this quick. I am very sad that my eel will pass but it came on so quick I don’t think there is much I could have done. Also other than the fish that have died and the tire track no other fish has this and the others that died didn’t have the skin stuff.

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Looks like a fungal infection

 

[latapy10]

Looks like a fungal infection

various infections-those are quite common in overstocked tanks...

 

[Cal Amari]

While I can't help identify exactly what it is if it's disease, bacterial, fungal viral, or parasitic, in any case when fish are dying frequent water changes of 50% or more is recomended, daily or every 2 days. If they have any chance of fighting it off they need frequent clean fresh water. Getting a test kit, or even test strips in an emergency would reveal some useful numbers. And all different fish have different immune systems, bigger fish are not always stronger or hardier than little guys. It really depends on the specific issue they are facing. Often scaleless fishes like loaches and catfish show signs of disease first. What kind of catfish is it and how is he doing?