Fishonly

Fishonly is a term used in the aquarium hobby to describe an aquarium that is stocked primarily with fish and does not include corals or most invertebrates. In marine setups, the concept is commonly implemented as a fishonly with live rock (FOWLR), which uses live rock for filtration while avoiding reef-building organisms.

In contrast to reef tanks or mixed community tanks that include corals or invertebrates, fishonly systems focus on fish behavior, compatibility, and easy maintenance. While simpler in some respects, successful fishonly aquariums still require careful species selection to prevent aggression, overstocking, and stress-related disease.

Setup and equipment for a saltwater fishonly tank typically emphasize reliable filtration, adequate water flow, and stable water chemistry. A protein skimmer, a marine biological filtration system, and a suitable substrate are common. Lighting can be moderate, and salinity is typically around 1.024–1.026; temperatures often range from 24–26 C.

Stocking and care center on compatible species and quarantine procedures. Provide enough space for large or aggressive fish, introduce new specimens carefully, and monitor for signs of stress. Regular water changes and testing of salinity, ammonia, nitrite, nitrate, and pH help maintain a stable environment.

Variants include freshwater fishonly tanks, which omit reef elements and often use standard filtration. The term broadly overlaps with FOWLR when live rock is used in saltwater systems. Limitations include reduced opportunities for coral keeping, fewer invertebrates, and potential conflicts among territorial fish.