Silver Jardini - Bubble like disease under bottom lip

Specs:
The tank is located in the living room
100 Gallon
tank setup 2 years
1 silver jardini
fish size 1.5 ft

Problem:
The fish hits bottom lip into tank with his mouth. (happens when people come near the tank) There was a little pink bruise (on bottom lip) that was caused by the fish rubbing against the tank. Now this pink bruise has turned into a big bubble on the bottom lip, which has grown in the past couple of days.

Pink bruise appreared about a year ago. The bubble occured a couple of weeks ago.

No previous treatments

All levels seem to be normal. When I say normal I mean the test that I bought from the store show normal signs. ( Ammonia, Nitrite, Nitrate, pH) I will check again.

There was a recent 20% water change.

I feed the fish pellets.. (rarely feed live feeders) food stored in closed lid jar. Feed once a day.

There are no plants a few small rocks under some gravel.

The fish is normal and friendly.. nothing changed about the fish. Just the pouch like bubble that has formed under his bottom lip.

Thanks for the help!

If you need to know anything else let me know.. Sorry no pictures.