Undergravel Filtration has been used for many years as the principal means of biological filtration in the marine aquarium, and despite the development of newer rival systems it still remains the most popular option.

Strengths of Undergravel Filtration

1) Universally understood and very widely available
2) Easy to operate, a bonus for beginners
3) Relatively cheap for smaller tanks
4) A natural environment for substrate dwellers
5) Quiet in operation
6) Calcium levels are higher and more stable due to calcium-rich substrate
7) pH is buffered by alkalinity of the substance
8) There is a large natural surface upon which bacterial colonies can thrive

Weaknesses of Undergravel Filtration

1) Displaces large amounts of water
2) Filter bed looks unnatural
3) Tends to clog with detritus
4) Bacteria consumes oxygen from the tank (but extra turbulence from the pumps may compensate)
5) Coral sand requires gradual renewal after two years
6) Calcium carbonates in substrate may interfere with effectiveness of medication
7) Collecting coral sand and gravel for the aquarium trade is less environmentally acceptable
8) Contamination of the tank by fragments of rusting metal collected from the sea bed with coral sand and gravel