Tim's Tropical Fish - Saltwater

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Cycling Saltwater Aquariums

Water Quality

Nitrogen Cycle
Nitrogen Cycle

After you've established the specific gravity, temperature, pH and dH,  it's time to understand the nitrogen cycle.

Waste produces ammonia which is very toxic to fish.  In an established tank, bacteria convert the ammonia to nitrites and then to nitrates, but in a new tank, beneficial bacteria haven't been established. 

An ammonia spike will occur within a day or two of introducing your new fish.  This spike can actually burn the fins off fish, ultimately killing them and causing great disappointment.  The most important way to keep ammonia levels low is to start with only a few fish or live rock and to add more after a few weeks.

Nitrogen Type

Reef Tank

Fish Only Tank


0 0


0 0.1


25 ppm 30 ppm

"Live rock" is material directly from an ocean and it has beneficial bacteria attached to it.  Similarly, "live sand" from oceans can also be beneficial.  The trick is to ensure the organic material hosted by rock and sand is still live by the time it is added to your tank.   Avoid material with algae on it.  If you have another saltwater tank, you can import material from it, including filter media.  For an interesting article about a fishless cycle and ways to balance an aquarium, visit the Tropical Fish Centre for info on the fishless cycle.

You can start by introducing mollies or other brackish livebearing freshwater fish, but you will need to remove them once you've established more aggressive saltwater fish in the tank.  Watch the ammonia levels carefully and don't introduce more fish until they are under control.  The level should be near 0 parts per million (ppm).  Cycling can take several weeks before the tank stabilizes.  Don't make the mistake of increasing the number of fish too quickly and losing an entire tank of fish.  It can take many months to stabilize a tank.