I haven't set up rainwater collection yet, but it is in the works.
The TDS meter is ordered.
I think I found the culprit sadly, I had bought a few coconut huts at a pet store, and instead of boiling them I just rinsed them in hot water... on a hunch I took them out and did a big water change this morning after finding 18 new deaths when I got up. there have been no new deaths so far today, but the huts did give off a weird film when boiled that I didn't expect.
That's right ^^^
If GH reading is at 12, TDS is going to be high too. That Nitrate reading of 20 will also contribute to TDS.
A TDS meter can be bought for cheap nowadays. It will confirm another water parameter. But you still need to do something about lowering the GH, and TDS.
@Linda, Do you collect rain water?