Will Fish Die If the Water Filter Is Broken?
Aquarium fish depend on filter and other life support equipment to keep them alive. If your filter breaks, you need to treat the situation as an aquarium emergency and replace it as soon as possible. That said, you can take some steps to buy yourself more time to pick up a replacement.
What Filters Do
Filters remove contaminants from aquarium water. The three types of filtration are biological, mechanical and chemical. In biological filtration, beneficial bacteria break down fish waste. In chemical filtration, materials like carbon or zeolite react with contaminants to absorb them and pull them from the water column. In mechanical filtration, debris is physically removed from the water column. Filters also improve oxygenation by moving aquarium water around. Without these processes, fish waste can rapidly build up to toxic levels and fish can rapidly use up their oxygen.
Preparing Your Fish
If your filter breaks, you should immediately take steps to improve your fishes' odds. First, stop feeding them. Most fish can survive several days without food, and fish produce less waste when they're not eating. You should also keep extra equipment, like replacement filters and cartridges, on hand if possible. A battery-powered aquarium air pump can provide aeration and run certain types of filters in an emergency. You should also remove any filter media from the filter and put it in a sealed plastic bag to keep it damp. Filter media can contain beneficial bacteria that will help keep a tank thriving. You don't want to throw these out along with your broken filter.
Replacing a Broken Filter
First, place the broken filter's filter media into the new filter. For example, some powerbox filters have biowheels or other filter media that you can swap from the old filter to the new one. This jump-starts the process of growing beneficial bacteria, helping the new filter function. You should also fill the new filter with water before plugging it in. This is called "priming the filter" and makes it easier to start.
It is difficult to estimate how much time you have to replace a broken filter, so you should always assume you need to act as fast as possible. Many factors determine how long fish can survive without filtration. For example, water at different temperatures holds oxygen for different lengths of time. A goldfish's temperate water will hold more oxygen than the tropical water most fish prefer. Additionally, some fish adapt better to filterless water—at least in the short run—than others. For example, fish in the betta/gourami/paradise fish family have an organ that allows them to breathe air if the water begins to get foul. Regardless, assume your fish are in danger and get a new filter as soon as possible.