alkalinity at 0! why?

[dmopar74]

ive tested the water in all 3 tanks!
ph-6.5
alk-0! why?
hardness-75
nitrite-0
nitrate-40
amm-0

im doing 50% w/c weekly, fish are healthy, water is cloudy in 90g other 2 are fine, ph is stable at 6.5 constantly, what gives?

 

[CHOMPERS]

Did you test your source water? Alkalinity is basically just a pH buffer; it keeps the pH from jumping around all of the time. Sodium Bicarbonate (Baking Soda) will raise the alkalinity. The Bicarbonate isons do the actual buffering. I am not sure if it is even needed or wanted in a fish tank. Water chemistry is one of my weak points in aquariums.