I've recently switched developer from perceptol to Exactol Lux. The developer requires an alkali stop. I've been using Alklai-STOP from monochromephotography.com and I'm getting purple negs wheather I use alkali fix or acid. Even after washing for 1.5hrs there is still a slight purple tint, I thought using alkaline chems shortened the wash time! I've tried presoaking and without. Same problem with both 120 and 35mm with FP4+. The documentation with the stop says 1L should be good for 60 films. I mixed 500ml and it lasted just 4 films. This is the only alkaline stop I can find in the UK. Are there alternatives? Anyone use it successfully?