Alkalinity TOOOO low

I have lost several fish in the last week due to what I believe may be the
alkaline being too low in my tank. I've done 2 water changes & even I've
tried to raise the PH a little in attempts to raise the alkaline. The tank
is 15 years old, however I've had it in storage for the last 5 years. I set
it back up 2 weeks ago. Here are the test results.

Nitrate (NO3) - 0
Nitrate (NO2) - .5
Hardness - 75 soft
Total Alkalinity- 0
PH - 7.2 neutral
Ammonia - .25 safe

I am using the 5 tests in one strip (Quick Dip) to test.

What I currently have in the tank is...
5 female bettas
2 black molly's
1 swordtail
3 green cory cats
1 Red tail shark
3 Black skirt tetra
4 White skirt tetra

It is a 55 gallon tank

Any suggestions?