Gh keeps dropping

[BeeBeeShrimp]

Hello all,

 

I am new to shrimp keeping and I have cycled my first tank.  I am using distilled water with a ph of 6.6 and re-mineralizing it with Salty Bee+ Premium.  My 10 gal tank has fluval stratum, driftwood, and a ton of plants. The filter has purigen.  When I mixed the salts with the water, I did so with a gram scale and per manufacturer directions to get a gh of 5-6.  The TDS was 110 with my particular pen.  I mixed my water gallon by gallon and each gallon had a gh of 6 when re-mineralized. To initially prepare my tank after cycling, I did a 90% water change with the new mineralized water.    After this change, I noticed that my ph was the same, sitting at 6.6.  My kh is 0.  But my gh has dropped to 3.  So, I mixed a strong solution of the Salty Bee in some distilled water in a small container and tried to add some more to the tank a little at a time to raise the gh.  I cannot get the gh to budge above 3, which I know is too low..but my TDS raised to 140.   

I am using the API Gh liquid test and measuring the drops it takes to turn the water a light green. 

What affects gh like this?  I highly doubt plants can lower it that quickly from when it was mixed...I am lost!!