I've just done another water test, the nitrate is 5.0. When I checked saturday I now realise that I stupidly misread the instructions and didn't shake bottle 2 for 30 seconds. Could the nitrate reading have anything to do with them swimming round.
Do 2 25% w/cs. So 50% total, or if you do large water changes regularly I would do a 50%. Then tomorrow another 50%.