If your testing strips are indeed giving you false readings then your fish could be suffering from Ammonia poisoning. What results do your testing strips indicate for Ammonia, Nitrite and Nitrate? Many people on here use API liquid testing kit many others use Salifert. I personally use API and have done for many years. The choice is yours really!
I would recommend however that because you are in a 'fish in' cycle you need to be changing a higher volume of water than 20%, around 80% I would think. You also need to be doing this daily.
Rushed out at lunchtime and purchased the complete API testing kit from nearest Pet Shop...got it home this evening and after reading instructions carried out all of the tests just to check against other readings...all tested safe...thank goodness.
However, money well spent much prefer this kit than the others I was using (no test paper or tablets). Much relief that the regime I was using was working for my setup.
However, I will proceed with the partial water change about 30% now and will keep monitoring water. Better to be safe than loose stock.
Will keep you advised.