At average,
ammonia level is 2ppm
nitrite level is 0.25ppm
nitrate level is 20ppm

It was a month old new tank, I had change 50% water twice a week but
the water quality always show the same. The plants didn't spread at all
after a month and some died.

My blood parrot fish had shown gill cover abrasion or swollen with rapid and deep breathing and I think it was nitrate posioning.
I had removed them from the tank and added tetracycline anti-biotic for medicine bathing, added methyl-blue, oxycure
and more air-stone for oxygen healing. But, the gills cover never had recovered and I put them back to poison tank again after a week of medication.

Could someone confirm with me that my nitrate is really too high and my fish is really nitrate poisoning?
My medication has no effect. What medicine could I use? I am trying to find ampicilin, but couldn't locate it.
Anyone know where to get it?

Worried a bit as the blood parrot fish had not been too healthy and the plant didn't spread at all to stop nitrate from posioning the fish.
Please advice me how to grow the plants successfully,

I had used seachem flourish, seaschem flourish excel, microbe-lift bloom and glow all-in-one and API leaf zone solution.
Could someone successful with aquatic plants advice me how they do it. Is it a neccessity to invest in Co2 system or
strong lighting to successfully plant their plants. What other things I had missed?