When bubbles are seen it is usually a line leak and it can be inside or outside the tank. It can also be a loose bung or crack in the tank. You can pressure test the tank before you open it up or after you replace lines by plugging the lines except for one, hold the tank under water and blow very hard on the open line and look for bubbles, that will show you if the bung and outside lines are OK or if there is a crack in the tank. If no bubbles then it's pretty sure it's an inside line problem.