Cardinal Tetras dying

[Kogi]

New fishkeeper here. I currently have a 75 Gallon tank.
Tank consists of -
3 Albino Corydora
2 Bronze Corydora
2 Candy Pleco
7 Dumbo Guppy
2 Glofish Tetra (It was rehomed to me, I will be getting more so they don't feel lonely)
5 Cherry Shrimp
and 11 Cardinal Tetras (was 17)

Water parameters attached below. PH is 7.5
Also attached a picture of two dead cardinal tetras

My cardinals have been dying one by one each day, and today I found 2 of them dead. I already started ICH-X and Maracyn together yesterday, and just dosed it today as well. I'm not sure what else to do, I have cleaned the tank glass, gravel vacced daily and raised the temperature to 84 yesterday when starting the dosing. (I didn't redose ICH-X since it says to do 1/3 water change before adding another dose after 8 hours, since the Maracyn says to not change water until I do more doses every 12 hours. (I'm assuming it's a fungal since I saw a white patch on the side of ONLY 1 of the tetra (not fuzzy though)
All my other fish seem to be doing alright, only the Cardinals have been dying. Any tips or anything I should do or look out for? This is a fairly new tank (3 Weeks old) but it was instant cycled with pre-used media and filter (from my previous tank (corydoras were doing great and had it for a while so I trust the bacteria in it), along with Fritz 700 Turbostart. Ammonia and Nitrite have never spiked (especially since it's a bigger tank with some small fish) I did tests almost daily to be safe. I'm out of ideas, and it's breaking my heart to see my little babies dying. As a newer fishkeeper I'm trying my best to read up on the most I can to do preventative and current health maintenances on my fish, and I feel like I'm failing. As for food I always gave them a mix of Xtreme flakes, freeze dried blood worms, and bug bites in a new salt grinder as well to keep a balanced diet. (Algae Wafers for the shrimp and Pleco). I did try to give a tad bit more food to assure everyone was eating, and I would gravel vacc 3-5 hours after feeding so no left overs could cause an ammonia spike. Sorry for the long read but I'm not sure where I'm going wrong. Anything will help.

 

[Colin_T]

Paragraphs with lines between them would help

How long have you had the different groups of fish for?
Some presumably came from your old tank and some are new?

How long have the cardinals been in the new tank?

Can you post pictures of the remaining fish?

How often are you doing water changes and gravel cleans on the new tank?
How much water do you normally change when doing water changes?

------------------
Cream or white slime/ film over the fish is excess mucous and is normally caused by something in the water irritating the fish.

Maracyn is an anti-biotic called Erythromycin and might have wiped out the beneficial filter bacteria.

Ich-x contains Malachite Green (aka Victoria Green) and Formaldehyde. This is toxic to people and fish but good for killing external protozoan parasites like Costia, Chilodonella, Trichodina, white spot and velvet.

The dead fish don't look like they have an external protozoan infection or a bacterial infection.

 

[Kogi]

Paragraphs with lines between them would help

How long have you had the different groups of fish for?
Some presumably came from your old tank and some are new?

How long have the cardinals been in the new tank?

Can you post pictures of the remaining fish?

How often are you doing water changes and gravel cleans on the new tank?
How much water do you normally change when doing water changes?

------------------
Cream or white slime/ film over the fish is excess mucous and is normally caused by something in the water irritating the fish.

Maracyn is an anti-biotic called Erythromycin and might have wiped out the beneficial filter bacteria.

Ich-x contains Malachite Green (aka Victoria Green) and Formaldehyde. This is toxic to people and fish but good for killing external protozoan parasites like Costia, Chilodonella, Trichodina, white spot and velvet.

The dead fish don't look like they have an external protozoan infection or a bacterial infection.

Only the cardinals are fairly new 3 weeks-1 month
I'm doing water changes every week, ranging from 10%-25%, kept an eye on my Ammonia/Nitrite/Nitrate levels daily. always <.25 Ammonia/Nitrite <10 Nitrate

The tetras I took a picture of does not have the white patch on its side, only 1 of the many that had passed had it. With that my LFS recommended I use Ich-X and Maracyn. Sounds like it wasn't the way to go. Although I did read up on bacterial/fungal infections, Gram Negative/Positive, since it's what the LFS told me to do I took it as he knew what he was doing (As I'm doing here).
Another member had mentioned to me it could also be my PH levels. I currently have 7.5 PH a bit higher than what cardinals are comfortable at but multiple sources including different LFS has told me as long as the PH remains constant they will adjust and be fine within that range. (Also bad info?)

So far only the Cardinal Tetras has passed, every other fish looks healthy and eats, cardinals also eat and swim very well. Hate to say but I'm not proud of what happened to my fish, since I enjoyed watching them for hours on end. With endless information, I'm trying to keep up as much as I can. More likely than not, I may have to rehome the remaining Cardinal Tetras if they survive to another LFS, until I get my PH in check(if it's the causing problem)

 

[Colin_T]

Can you post some pictures of the fish you have in the tank so I can check them for diseases?

A pH of 7.5 is not going to be an issue unless the cardinal tetras are wild caught. The shop should have had a sign on the tank saying wild caught if they were.

Perhaps call the shop and find out what the pH and GH of their tank water is. If it is drastically different to your water, that might be a contributing factor. However, if the pH and GH of their water is similar to yours, then water chemistry is unlikely to be the cause.

Having said that, it is unlikely to be an issue with the pH if this has been going on for 3-4 weeks.

When did the fish start dying?