The crossword solution for the term BUILDUP AQUARIUM is ALGAE, which is a type of aquatic plant. ALGAE are found in both freshwater and saltwater environments, and they are an important part of the food chain in many aquatic ecosystems. In aquariums, ALGAE can help to create a balanced environment by providing oxygen and nutrients to other organisms. They can also help to reduce the buildup of harmful substances such as nitrates and phosphates.