Looks like it could be a Thyroid Tumor. It can be treated with iodine for a few weeks. Try using potassium iodide and iodine. You should notice is residing between 2-4 weeks, if not, it could be a malignant tumor (no treatment).
5 g of potassium iodide
dissolve those into 100ml [100x.003=3 fluid ounces]of water (sorry, my book was written based on metric and a mathly person will have to translate to Gallons)
add 1 ml [1x.03=.03 fluid ounces]
(of the solution) per 50 liters [50x.26=13 Gallons], don't run carbon.This should be added after each PWC
This book has a metric converter, someone check my math!!
Basically, mix .5g iodine 5 g of potassium iodide in a 3 ounce or larger container, add .43 ounces to your tank, wait a week, do a PWC
and re-add the next bach, keep this up for 2-4 weeks.
Here is my math for the amount going into the tank
185 total volume/13G=14.23x.03=.426 of treatment