Yeah, mainly to reduce nitrate. Gets really high in my Frontosa tank and only can be diluted with water changes. Tanganyikan Cichlids are very sensitive to high nitrates so quite big water changes need to be done weekly.

Same with the marine tank, corals cant take high nitrates and slowly die. No other effective way of lowering it.

__________________
8' x 2' x 2' - Marine Reef.
6' x 2' x 18" - Wild Caught Zaire 'Moba' Frontosa
4' x 2' x 20" - Malawi
4' x 22" x 18" - Malawi + Victoria
4' x 18" x 18" - Breeding Cuban Cichlids.
Trigon 190 - Male Paratilapia Polleni
20" x 20" x 20" - Marine
30" x 1' x 1' - Hospital Tank
2' x 1' x 1' - Tropical Community

and

7x - 1' and 2' cichlid fry tanks