Why is my aquarium water cloudy

Despite thorough rinsing, some particulates might still arise from sand/gravel or decorations. These can cause some initial cloudiness as things settle and any persistent issues can be quickly solved by the use of a flocculant, which clumps these microscopic particles together allowing them to be removed by the filter.

Unlike any other type of pet keeping, an aquarium doesn’t thrive when freshly cleaned out and sterile but requires a balanced population of bacteria to function. When newly set up, things can be a bit cloudy until the right bacteria are present, and sometimes a bacterial bloom will occur. As the filter matures and competition for nutrients subsides, the population of microbes will usually keep the water clear and clean.