Everybody poops. Even your fish.
To keep your fish safe you need two things: Beneficial bacteria and waste removal.
Waste removal is simple. “Dilution is the solution to pollution”. Performing regular water changes dilutes and removes waste. Plants can help too as they’ll use the waste to grow.
Establishing a beneficial bacteria colony is a little more complicated and requires patience. Fish keepers often call this process ‘cycling a tank’ because the bacteria are participants in the nitrogen cycle (which is one of the cornerstone “biogeochemical” cycles that supports life on our little planet).
In the aquarium, we’re looking for bacteria that convert highly toxic ammonia (fish waste) to less toxic nitrate to buy you time between water changes. You can allow nitrate to build up to much higher concentrations before it is harmful to your fish than you could do with ammonia.
For context, ammonia is toxic at only 0.5 parts per million (ppm). Nitrate can be safe up to 100 ppm, depending on your set up. Each ppm of ammonia turns into just over 3 ppm nitrate (for reasons related to the units), but that still gives you LOTS more time before your fish are poisoning themselves.
There are nearly as many methods to ‘cycle’ a tank as there are fishkeepers, but ultimately, every method does two things: provide food for the bacteria and time for them to replicate.
At Indoor Fins, we suggest a version of ‘fishless cycling’. To do this you add small amounts of fish food every few days for a few weeks. This feeds the bacteria. You can speed this up by adding live plants from established aquariums or getting dirty filter media from a healthy tank. These both bring with them some of the bacteria you need to jump start the process. Beneficial bacteria mostly live on surfaces, not in the water. So, dirty water won’t help much. Plants, filter media, sand/gravel, or decorations will have more beneficial bacteria than water.
The best to know your tank is ready is that you can detect nitrate in your tank (or if your water source has nitrate, then you detect more in your tank than your source water). Test stripes and liquid tests will work for this. Liquid tests are more precise and accurate, but also more expensive and time consuming.
If you can’t test (or don’t want to), waiting a few weeks and seeing algae growth (especially green algae) is a good indication that your tank is ready for a small amount of ‘bioload’ from fish. Over the months as your tank gets better established you can add fish a few at a time.
And that’s it! Once you have a thriving colony of bacteria you are off to a great start to successfully keep fish. Happy fishkeeping!