TDS

As a follow up note on my previous post; I changed half the water again (rain water) and the TDS reading rose on my meter. In unbelief, I tested several other tanks. On a couple of them (ones that I would have suspected of being higher than the rest), I get a very low reading with what looks like a very small 'e' in the upper right corner of the display. The instructions do not say anything about this. I now suspect that instead of the fish going from low water to high, they may have gone from very high to high. Does anyone know what the symbol on the display means?