Thanks reefman :stonight I usually remove a couple of litres in a jug add more salt then put it back in the tank. I'll leave it tonight (uk time LOL.) and add some more in the morning. I found an old wine hydrometer I used to use when making wine a few years back, this is calibrated at 20c though, so using a conversion table to get it to the right temp it comes out at about 1020/1021.
regards
Dave