Betta Fish - ? tumor

Question
I have had my betta for 2 years.... I keep it in a glass bowl
My betta developed what used to be a small pebble shaped growth on its tail about a month or two ago. I change its water about every three weeks, add 1/2 new water about once or twice in between, it is near sunlight and seems to like it , he is still aa active as ever and his appitite is still very good. The growth has done nothing but grow...it used to be on only one side of the tail...but now its on both...almost like it was pushed in the tail half way or something. I first thought it looked like a peice of food had gotten lodged inbetween the tail. Its about the size of a pea  Ive tried to look this up elsewhere on the internet but I have yet to find something similar to what my fish has. All other diseases I have read about dont match the desciption on symptoms that my fish has. Please help! I dont know waht exactly this thing is or how to treat it, or else I would have long ago. I would greatly appreciate it if you could give me some insight as to what this is and if it can be treated

thank you!!  

Answer
Hello Maureen,
It is possible that your fish has Lymphocystis Virus, which is untreatable. It resembles a small cauliflower like growth on or around a fish's tail.
In the future, you need to do more water changes to prevent this from happening. You should be changing out the water once a week, not once a month. Build up of toxic chemicals such as ammonia, nitrates, and nitrites will occur, and having prolonged exposure to these chemicals will eventually kill your fish. It will suffer in the process, and being that fish hide their illness you will never know it until it's too late.
So make sure you keep their water fresh, clean and dechlorinated at all times, and ammonia levels should never go above 0.
It is also a bad idea to keep a fish near a window, since the temperature will constantly be changing, which can also stress out a fish to the point where they are weak against disease. Not to mention a bowl of water can quickly overheat.
Hopefully that will help you in the future. For now, clean out your betta's water asap and take him away from the window, put him in a one gallon tank (you can get one for under ten dollars) and buy him a small heater.
You can also add some aquarium salt to help him replace his slime coats. Make sure you use "Aquarium Salt" - not table salt.