I bought my betta fish about a week ago from Petco. He's in a 1.5 gallon tank with a log and gravel and a live plant and a filter. Yesterday night, he developed a huge lump underneath his mouth that I've never seen before...He hasn't been eating, although I've offered him pellets. I haven't changed his water yet, because I'm scared he'll get even more stressed out. The tank is a bit cold (70 degrees) but I have a Marina C10 heater being shipped right now. Should I change his water? Is the lump a tumor or something? If I have to change his water, should I do a 100% change?
I'm really worried...