Many non-Japanese conflate sashimi and sushi; the two dishes are actually distinct and separate. Sushi refers to any dish made with vinegared rice, and while raw fish is one traditional sushi ingredient, many sushi dishes contain seafood that has been cooked, while others have no seafood at all.
It's the moisture and contaminents that break down the DOT 3 and 4 fluids which is the reason to change the fluid in the first place. If you don't change it, what happens is moisture enters the system via the vented fill caps on the brake master cylinder. This moisture along with dust, mixes and travels in the system eventually making it's way to the lowest points in the system.
Unless you have a leak in your hydraulic system, you should not have to add brake fluid. The fluid level sinks, however, as your pads wear. That's because with worn pads, your brake pistons are further out of the caliper. Once you force them back with new pads, the fluid level should go back to its original level. In other words: If your fluid level is low, check your pads!