A pool store has told us we have high Total dissolved Solids and this is dangerous and we have to empty half of the pool water?

Having a high or elevated TDS can cause the water to look flat or have a salty taste, it can also promote the formation of scale on the finish. If left unchecked the scale that forms on the finish will require professioanl treatment to remove it if indeed it is removable at all.

TDS is the measurement of all of the dissolved organic matter and solids in the pool. If you have a salt chlorine generation system you should deduct your salt level measurement from the TDS total to reflect the correct TDS reading.

TDS that is elevated can only be reduced by draining your pool and refilling with fresh water, ideally NOT well water. The amount you will need to drai isdependant on your final TDS reading.

Ensure your TDS remains between 1000ppm and 2000ppm. 3000ppm is the absolute maximum. If in doubt about how much water to dump obtain advice from a pool professional. The general rule is dump 12 inches then retest, if necessary dump another 12 inches then retest and continue until TDS is in range.