Can't keep neons alive

[CraigS]

Good afternoon all,
I've been out of fish keeping for quite some time and have recently got back into it.
I was gifted 120 gallon tank and have had it up and running since May. It is scaped with Eco complete , Amazon swords and Anubis, mountain Stone and spider wood.Substrate was treated with seachem flourish. Temperature is kept at 78°
I made the mistake of adding too many fish after the first month even though I used seachem stability and very sadly lost most of them.
I have did 20% water changes once a month since then and I'm using seachem prime and fluval cycle with each change.
The fish who remained are one yellow laser Corey, two bleeding heart tetras, and four otto catfish.
3 weeks ago I had my water tested and all showed good so I added 10 lemon tetras who are doing great but at the same time added 10 Cardinal tetras who did not look the healthiest when I purchased them and all died within a day.
This weekend I added five bleeding heart tetras and so far are also doing great but also added eight black neon tetras which I heard are more robust and one didn't even make the ride home the others looked healthy but I'm down to only one the rest have died.
My last water change was 2 weeks ago.
I bought an API master test kit and have tested my water before my last fish purchase and just today again.
The parameters are as follows:
PH 7.6
High pH 7.8
Ammonia 0
Nitrite 0
Nitrate < 5ppm
I running a fluval fx5 with full length spray bar. Inside I have ceramics, sponges, polishing pads, carbon, seachem purigen, and ammo-carb.
Any advice would be greatly appreciated, am I doing something wrong or not buying healthy fish.
I know the water is on the hard side and the pH on the high side as I am using tap water.
Eventually I would love to have a couple of discus in the tank , I know there are ways to lower the ph and the hardness but want to be sure that my tank is healthy before going down that road or is that actually the problem with my little neons, cardinals and black neons.
Thank you all

 

[Aiken Drum]

Neons and cardinals natural habitat is soft, low pH water. But they are adaptable fish and can live in a wide variety of water conditions. If you bought the fish locally, the likelihood is they were bred and raised in similar water to whats coming out of your tap. If you are buying wild caught fish they are going to be less tolerant to your tap water. Its unlikely they are wild caught but may be worth asking whoever you are buying from.

The most likely cause is you are buying unhealthy fish. But it could be an acclimating issue. How do you acclimate your fish to your tanks water before introducing them?

Because you have mentioned discus im going to urge caution. These fish arent as tolerant to varying water conditions, and generally do prefer soft low pH. Yes there are ways to alter water parameters, but you would need to alter them a lot. The non chemical ways of adjusting things wont cause that much movement and chemically altering things usually causes more issues than it solves. If you did want to go for discus i would consider RO/DI water which can be remineralised to the specific needs of your fish. If you are going down that route i would start out with that in mind rather than trying to change things some time in the future.

However. In the UK one of our major aquarium store chains stock specially bred discus that can be kept in tap water. Maybe thats an option for you too in your location.

 

[CraigS]

Allow the bag to float for 30 minutes then add some tank water for 15 minutes before releasing them using a net so not to chance store water from contaminating my tank.