Cheers again guys. I think I might not be giving them enough of a change all at once. When I've took the TDS from 160 down to 80, it's been through a series of waterchanges over the space of a week or so, lowering it by 10 to 20 PPM each time. I might need to shock them into a spawn, lol.
I found the TDS meter a handy little gadget to test nitrates for tanks needing water changes. If the TDS level rises a large margin you can bet a nitrate test will show thats risen as well.
Higher pH will raise the TDS level as well so best to make sure of the regular TDS level relative to the pH first before assuming nitrates are high.