Ammonia is produced by fish in the aquarium. Raising ammonia levels are very harmful for the fish and can even cause mortalities.
Check amonnia values regulary to check if your filter is performing well. Especially during the start of a new aquarium and after a thorough clean-up it is very important to check this value frequently.
Colombo have produced test kits that are developed especially for freshwater. These tests give the best results!
Colombo, 30 years of experience, that makes the difference.
