Seems to me there's something inside the tank absorbing the KH.
One of the biggest culprits is often high CEC substrates such as Safe-T-Sorb and related materials, and several of the ADA substrates.
I have not seen that with EC.
Organic processes can break down the carbonates. Some microorganisms use the carbon from carbonates. It takes a long time with no replacement (water changes) before the KH will start dropping from this, though. (You can do more research using the term Old Tank Syndrome) It sure does not happen in a week! More like several months to a year.
As noted above plants without added CO2 can start to use the carbon from carbonates. This also will not drop the KH from 5 degrees to 0 in a week.
Maybe try this:
Set up several glasses of water, and treat each one in a different way. Each one gets something from the tank, but only one thing.
One glass gets a handful of substrate.
One gets a bit of rock or driftwood (if you have either in the tank).
Another gets only what ever fertilizer you are using.
Another just gets some dechlor.
... and so on, until you have tested everything you are using in the tank.
Run each test for a week, and test the KH perhaps mid-week, then at the end.
Here is a way to make the KH test more sensitive:
Use only half test water, and half RO water. (test the RO to be sure it really has zero KH).
When you count drops to see what the KH is, count like this:
One and a half
Two and a half
... Each drop is half a degree of KH.