Help! Everything in my tank keeps dying after getting cardinal tetras (TW - Dead fish pic)

[lukesfishies]

Hi everyone!
I'm posting this again as the previous one seemed to disappear.

Quick backstory:

I'm quite new to fishkeeping, but I've made an effort to do everything correctly and I've done a lot of research on the right procedures.

I have a ~30gal tank, and after it was fully cycled I added 6 guppies to it, a few weeks ago. The guppies are very happy and have grown a fair bit, and have become very colourful and active. This Saturday, I added 3 Pakistani loaches, 4 assassin snails, and just over 10 cardinal tetras. Ammonia is 0, nitrite is 0, nitrate is between just over 10ppm, the pH is constant at between 7.4 and 7.6 (a bit alkaline I know), and the water in my area is slightly soft. I did a water change the day before getting the fishes (Friday). I also have two airstones and the tank is fairly planted.

OK now onto the problem:

Yesterday night, one of the tetras was floating around vertically, swimming around occasionally. I took him out and quarantined him in a fishbowl with a heater and water from the tank. He floated around, but lived for a bit, then died sometime during the night. His tailfin was mostly destroyed, but the others were fine tail wise.

A few minutes after quarantining that fish, another one started floating vertically and died instantly seconds later. All the tetras stopped schooling. Today, everything was fine and they're schooling again. The loaches and guppies are still happy. Then tonight, another tetra started floating vertically. I took it out and it was already dead, and it had a pretty foul smell. The stomach area was soggy and opened very easily which was weird. The tetras are still schooling. I've attached a pic of the last dead fish, and of the rest of the tetras. Now that I took the pic they actually do look a bit discoloured, but it is 10pm so it might be because it's night time? They otherwise look in good shape (until they suddenly drop dead that is).
I acclimatised them in their bag over maybe 30-40 minutes by adding tank water bit by bit.

EDIT: added a video:

I tried to get as much as I can but it was a bit confusing lol. The dodgy fishy is at the start and end of the video. He's not dead yet but he's being similar to the others that died, but the others were more vertical and didn't swim as much

UPDATE: 5 tetras have died so far, all in the same way. with the last one I saw a large colourless spot the size of its head on its side. Now this evening, one of the strongest guppies died, who showed no weakness before. His left fin was stuck sideways and he kept trying to trap himself in plants or between the filter and the tank glass. He died within minutes. I gave the guppies and the tetras a ~15min salt dip.
The guppy has a dark area around his head.

Before this, during the day today, I did a ~60% water change, vacuumed the gravel, added fresh gravel, bought and added some stresscoat+ and API melafix. New water readings, ammonia and nitrite is 0, and nitrate is less than 5ppm

Thanks for any help!

 

[seangee]

What is the actual hardness of your water. We need numbers and units, these may be available on your water supplier's website. Water that is good for guppies is not good for cardinals as their requirements are very different. The dead fish has had a bite taken out of it but this may have happened after death, in which case it doesn't mean anything. The live fish look very pale, which may suggest stress or water quality.

Just FWIW cardinals are not schooling fish. When they all gather together in a tight group it is because they are stressed or feel threatened.
edit: Those are cardinals in my signature pic - if you look closely you will see they are pretty spread out.

 

[lukesfishies]

What is the actual hardness of your water. We need numbers and units, these may be available on your water supplier's website. Water that is good for guppies is not good for cardinals as their requirements are very different. The dead fish has had a bite taken out of it but this may have happened after death, in which case it doesn't mean anything. The live fish look very pale, which may suggest stress or water quality.

Just FWIW cardinals are not schooling fish. When they all gather together in a tight group it is because they are stressed or feel threatened.
edit: Those are cardinals in my signature pic - if you look closely you will see they are pretty spread out.

I measured the TDS with a TDS meter and it reads 84 PPM, so a bit soft.

 

[lukesfishies]

from the water supplier:

Typical water hardness: Soft
Hardness clarke: 2.24
Date of update: 19/06/2020

 

[seangee]

That water is fine for tetras and loaches. It is too soft for guppies and possibly for the snails but those are less urgent issues. I would do a 75% water change to rule out any contaminants.

Do you have any pictures of the guppies - I am no expert on guppies, but they should not be aggressive enough to bully tetras to death. But not everything sold as a guppy is always a guppy.

 

[JuiceBox52]

I agree if it is a guppy it wouldnt be that aggressive

 

[lukesfishies]

That water is fine for tetras and loaches. It is too soft for guppies and possibly for the snails but those are less urgent issues. I would do a 75% water change to rule out any contaminants.

Do you have any pictures of the guppies - I am no expert on guppies, but they should not be aggressive enough to bully tetras to death. But not everything sold as a guppy is always a guppy.

The guppies are in very good shape and they haven't been hostile at all, they've been very friendly and peaceful towards the tetras. The fish don't get bullied, they just start chilling vertically out of nowhere (and act a bit weirdly, mostly still, but slightly erratic at times) and a few minutes later they're dead.

 

[lukesfishies]

Here's a pic of the guppies. The paperclip fell in and they got super curious lol

 

[PheonixKingZ]

Floating usually indicates a problem with the swim bladder. What are you feeding them and how much?

Edit: If you are feeding them flake food, this may be the problem. When fish come up to the surface to grab a flake, they also swallow some air. This causes them to float. (Although, this doesn't sound like the problem, as you were just starting to add them)

 

[lukesfishies]

Floating usually indicates a problem with the swim bladder. What are you feeding them and how much?

Two pinches of flake food once or twice a day, plus some live bloodworms on Sunday instead of the flakes. The guppies eat much more of the flake food so the tetras don't eat much, although they did have more bloodworms

 

[lukesfishies]

They don't really float though, much of the time they're just vertical at around the middle of the tank, and they spin around themselves a bit

 

[PheonixKingZ]

They don't really float though, much of the time they're just vertical at around the middle of the tank, and they spin around themselves a bit

We really need a video to help you. Please upload one through YouTube and post the link here.

 

[lukesfishies]

One we're keeping an eye on now is swimming a bit, but mostly at a 45° angle, by the heater. I put him in a bowl of tank water to separate him, then he seemed normal, so I put him back after about 10 minutes, into the other end of the tank (it's a 4ft wide tank), and he swam straight back to the heater and stayed there

 

[lukesfishies]

I tried to get as much as I can but it was a bit confusing lol. The dodgy fishy is at the start and end of the video. He's not dead yet but he's being similar to the others that died, but the others were more vertical and didn't swim as much

 

[Retired Viking]

I agree with @seangee and advise you to do a 75% water change and vacuum the gravel. Guppies are more hardier than cardinal tetras so there may be something in the water affecting them more than the guppies. Try to always wash you hands before feeding or putting them into the water. Have you added anything to the water like stress coat or anything else?