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.
Adding brake fluid is usually not part of routine vehicle maintenance. Low brake fluid typically means that your brake pads are low and soon need to be changed. If you notice a sudden drop in the brake fluid level, it could indicate a problem with your brake system, or that you simply need to change your brakes. Have a professional check your brakes if you are unsure.