Firstly great to see its reducing in size!
GOOD NEWS! Its not a tumour its a lymphocystis. This is an infectious viral disease that causes enlargement of cells (hence its tumour like appearance) Once infected there is no treatment it will naturally run its course in around 4 weeks. Lymphocystis rarely kill the fish its usually secondary bacterial/fungal infections that pose a problem with this condition. What have you been using to treat? I suspect its that treatment that has saved your fish so well done for your quick response! Unfortunately, there is no real way of ridding it from your tank. The 'bumps' you saw where enlarged cells acting as a 'viral factory' which will then release the virus into the tank. Are there any other fish with it? What you can do it completely dry out the tank and substrate/bogwood and leave it for 7 days this MAY work. Also there has been cases where the virus has infect humans, but don't worry our immune system can handle that. If infect you will notice worts/bumps on your hands or arms, just see the doctor and he will give you some antibiotics.
Hope this helps, you have been doing exactly the right thing! If you could tell me how you treated this/ husbandry that would be really helpful for further studies.
Thanks and well done again! You zebby is very lucky to have you!