Jumpo size XXL cardinal

[warrenlee]

Good Morning everyone,

I have been in this hobby for 1 year.

Recently, I developed a keen interested on
realing a big cardinal, after seeing the
beautiful species swimming around, with
its deep red color.

My tank is 3ft matured, have been in for
almost 1 year and I have keen only 80
small cardinal and 30 big cardinal, 10
Harlequin Rasbora, 8 Pink danio, 2 zebra
danio plus some cherry shrimps, 2 otto, 10
yamato.

My 3ft tank parameter:
GH: 60
KH: 40
PH: 6.8-7.0
No2: 0
No3: 20
C02: 3 bps.
temp: 26-28 with fan (no chiller)
Eheim Pro II 2028

planted with java moss wall, driftwood
with us fisidden and mose.
foreground: japanese hair grass and glosso

My problem is with big XXL cardinal.
I have researched and read from internet,
they said Cardinal Tetras needs lower ph
like 5.5 -6 PH ?
Do I need to lower my PH ?\
any suggestion ?
I bought sera super peat and put inside my Eheim Pro II 2028, but PH is still hovering 6.8-7.0
I have bought XXL cardinal but have not
been able to keep successfully, it does
about 1-2 weeks then die off, with tail
fin rot, a batch of 30 pieces for $50. And
I have not given up, and I tried again,
and it dies again, totally loss $100.
Then, I thought it is because it caught
wild so, it dies off easily. and a 3rd
time, is when I visited a rainbow fish
farm, and saw their big XXL cardinal also.
They told me their XXL cardinal (6 cm
long) are kept from small, not a wild
caught. So, its adoptability and survival
rate is higher. They are expensive, cost
$2.5 each. I have bought another 10 pieces
(dare not anymore, only 10 pieces). But
again, it slow die off in 2-3 weeks with
same symptom : with tail fin rot.

Sigh....? thinking of giving up the hope
of have big XXL cardinal.

but my small cardinal and others fishes
and shrimps are doing fine.

What wrong with those XXL cardinal ?

please kind hearted soul, please advice ?

Thanks,
Warren

please kindly advice ?