If you are using the api tester, 8ppm is as high as the colour chart goes. So if you have 20ppm or 100ppm ammonia it will still show as 8. The trouble with high ammonia is that at concentrations of 8 or above, the wrong species of bacteria starts to grow, that's why you need to get the reading down well below 8. It sounds like you need to remove most of your water (though with that volume I can understand why you'd be a bit reluctant). Then when you've added the new water test again and add more ammonia if necessary to get 5ppm.