What do I do now (with these reading)

I have been taking out 1/4th of the water in my 20 gallon tank for the last
week. The four remaining fish seem to be doing well (After the quick die off
of the other 4) and have been taking readings twice a day. I am not changing
the water again until Sunday (When I have time). Here are my current
readings as of 20 minutes ago. I used Mardel Freshwater Test Strips.

Nitrate-20 ppm (Safe)
Nitrite-0 ppm (Safe)
ppm total hardness - 50 (Moderate)
ppm total alkalinity/buffering capacity-240
ph -8.4

What now?

Also I have a fern type plant in the tank and it seems the four remaining
fish are eating it down to the nub. Is this normal? Also the fish are eating
what I put in the tank and what remains on the bottom they eat off and
on..normal?

Can I put Tank Buddies ph leveler tabs in the tank with the fish in thier?
The box doesnt say one way or another.

TIA
Jeff