PH DROP???

[Braddah]

So I just lost an angel in my 20 gal.... she started looking unwell the other day so I tested my water quality....No2 0ppm Ammo 0ppm and pH below my scale Id have to say its at about 5.5-6.0.....so I added some proper pH 6.5 (I did a water change first). Well my water is still acidic and I cant figure it out (the water in the tap is about 7.0) its like this in all my tanks. Any ideas about what could cause a pH drop like this?

 

[reg2k2]

Test your KH (carbonate hardness)...too low, and it can cause the pH to 'crash'.

Crushed coral in the filter (in mesh bag or nylon) is one solution.
Sry for your loss.