High ph low kh

[Jason A.]

So I've started to loose my cardinals I've noticed my ph is a little high around 7.4-7.8 and I know my kh is little light, had a hard time getting my nitrite to cycle till I raised my kh and now I'm losing 1-2 cardinals every couple of days . I have video of one of my cardinals floating across the substrate and twitching and trying to live but ultimately died, I don't know how to post the video here. I see no signs of illness in my cardinals or other fish and other water parameters look fine. Any advice and suggestions are welcomed. Thank You 

[Wingman12r]

Did the tank complete it's cycle? Nitrite poisoning would better explain the death.

[Jason A.]

Yeah it did

[Jason A.]

Just checked my water again and all is good but ph and I did my kh test again and seems to be 0-50ppm but when I test my gh it doesn't register, I've added as much as 30 drops  and no reaction, my tap water takes 6 drops. Tomorrow I'm going to do a 25% water change I'm hoping that won't stress and kill the rest of my fish