Fish tank buildup is a common problem for aquarium owners. It is caused by the accumulation of SCUM and ALGAE, which are both four and five letter words respectively. SCUM is a thin layer of organic material that forms on the surface of the water, while ALGAE is a type of plant-like organism that grows in water. Both SCUM and ALGAE can cause fish tank buildup if left unchecked, leading to poor water quality and unhealthy fish.