High NO2 (Nitrite) reading and low PH

[Toptex]

Checked the water tonight (did a 50% water replacement yesterday) and got a low ph - 6, and 0.25-0.50 Nitrite reading. Ammonia and Nitrate were 0. Why? Decided to do another 50% change, another full water test revealed the same reading.
What is my next plan of attack to get the perfect reading?
FOI - sand substrate, only one axie, 60cm x 30 tank and temp, around 20-21C

 

[SludgeMunkey]

Any wood in the tank?

How long has the tank been set up?

Which water pretreatment are you using?

Any plants? live or artificial?

Is there, or has there been heavy algae growth in the tank?


Finally, what caused you to do such a large water change to start with?