Filter media for TetraTec EX range and other external filters. Highly active biological filtration. Large surface area (62cm2 per ball) for growth of beneficial bacteria Excellent water flow through balls prevents clogging and \'dead\' areas, for maximum biological efficiency Easy to clean with aquarium water Rinse in lukewarm water before use. Suitable for all external filters. 800ml 1 pack is sufficient for the TetraTec EX 600 & EX 700 2 packs are sufficient for the TetraTec EX 1200 3 packs are required for the TetraTec EX 2400