How to Turn a Fresh Water Tank Into a Salt Water Tank

Very few ornamental fish can survive in both freshwater and saltwater, but some aquarists enjoy the challenge of converting an established freshwater ecosystem into a brackish or saltwater environment. This conversion may be necessary for species like the green spotted puffer, which can thrive in freshwater only during the juvenile stage. In general, it is easier to begin with a new fish tank than to convert an established aquarium to a new salinity level. The process of significantly converting a freshwater aquarium into a saltwater aquarium is time-consuming and can be extremely stressful for fish. Still, pre-established freshwater tanks offer the benefit of providing a full population of nitrifying bacteria and stable nitrate and ammonia levels.

Things You'll Need

  • Established freshwater tank
  • Saltwater-tolerant fish
  • Nitrate test strips
  • Ammonia test strips
  • Marine salt
  • Hydrometer
  • Appropriate decorations
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Instructions

    • 1

      Verify that all fish and invertabrates in your aquarium can tolerate saltwater. Archerfish, sailfin mollies, black mollies, figure 8 puffers, green spotted puffers, bumblebee gobies and American flagfish can all tolerate saline water to varying degrees. Using a credible resource, confirm your the salinity tolerance of each fish in the aquarium. The specific gravity (salt level) you choose as your target should reflect the needs of all fish in the tank.

    • 2

      Test nitrate and ammonia levels in your aquarium twice weekly for at least a month, documenting any changes. If your aquarium is well-populated by nitrifying bacteria, there should be no significant fluctuations. Do not alter the salinity of the tank unless the nitrate levels are consistently below 20 parts per million (ppm) and no detectable ammonia is present. If your aquarium experiences drastic changes in chemistry on a frequent basis, wait a few months before attempting to alter its salt content.

    • 3

      Add small amounts of marine salt to the aquarium during routine water changes. This can start with as little as one half teaspoon per gallon--the slower, the better. Gradual changes in salinity minimize fluctuations in nitrate and ammonia; they also enable the fish to adapt with less stress. Blend the marine salt with the "new" water before adding it to the tank; this ensures that the salt is distributed evenly throughout the aquarium.

    • 4

      Using a hydrometer, monitor the specific gravity of the aquarium water during twice-weekly checks of water parameters. If the levels of nitrate and/or ammonia begin to climb, wait to add more salt to the aquarium until the levels have re-stabilized. Continue adding marine salt gradually until your aquarium has reached its target salinity. After that, replace only salt lost during water changes.

    • 5

      Remove snails and plants from the aquarium as soon as they begin to weaken. Most freshwater snails and plants will not survive in saltwater, but the degree of saline-tolerance varies by species. Over time, replace freshwater ornaments and plants with decorations that are compatible with a saltwater ecosystem. A saltwater ecosystem is most likely to thrive with fine substrate and live rocks.