How long can a Saltwater Tank go without a filter?

A saltwater fish can live without a filter for a maximum of seven days and a minimum of three days. A filter helps clean the water over and over again, so the toxins in it don't settle down. But that doesn't make the filter a necessity for fish survival.
Your fish can survive in an artificial environment without a filter for about 3 days to a maximum of a week. The number of days totally depends on varying factors such as the number of fish, measures you are going to take to ensure filtration, the resources you have, and alternatives.

Functioning of a filter in an aquarium

An aquarium filter performs mainly two basic functions- cleaning the dirt and debris from the tank and flowing oxygen.
When the filter performs cleaning actions, that process is called a cycle. When you attach a filter, the oxygen circulates in the entire tank, and when the filter stops, the circulation stops as well since the oxygen supply is only available at the bottom-most level, fish begin moving towards the surface to breathe. Canister filters work quite well in saltwater aquariums.

Keep your fish alive and healthy without a filter

When you can't spend the required money for a filter and you need your aquarium to work without it. Following are the ways your fish remain alive and healthy without a filter-

• Put live aquarium plants instead of artificial plants, live plants help in converting Carbon dioxide produced by your fish into the oxygen they need to breathe, it acts as a filter.
• Put beneficial bacteria in your aquarium, it boosts beneficial bacteria populations which improves bio-filtration and reduces waste.
• Perform water changes that reduce excess nutrients, control algae growth, and remove tannin (color) producing matter.