the purple thick slimy gunk is more then likely Cyno Bacteria, it reacts like algea but it isn't. My battle with it was easily won, cut your light cycle down your animals need it too so it can't be totally absent. I went with an 8 hour lights on cycle, the other thing is high nutrients in the water, small water changes and lots of them. I was doing 2-3 10-20% changes a week. Most important when syphoning the tank suck up all the gunk ya can and export the nutrients...it's in the gunk.
This is a bit of work I must admit but it's the safest way to get it all out I have found and only took a few weeks. After the battle was won I never had a problem again... mostly because I lowered the ammounts I fed my fishes when I remembered all the work I would have to do all over again if I didn't
