I am definetly not an expert on this subject. However, whether you are planning a new tank setup, or just trying to keep your existing tank water in healthy condition, having a basic understanding of the nitrogen cycle can help. This info might be helpful for many freshwater tanks, but I will mainly discuss the theory behind my setup in relation to the nitrogen cycle. The image below is a very basic diagram of a planted tank. Notice that the fish and decomposing matter in the substrate produce ammonium. Also the plants and nitrifying bacteria help convert the ammonium, nitrites, and nitrates.

  Now, there are a lot of people who keep arowanas in a bare tank. There are probably several reasons why this is so popular. I think it started with owners of expensive asian arowana and the fish was the only thing important to see and any other objects in the tank just distracted from the fish. Plus any objects in the tank could cause injury to their very valuble fish. So, a empty black tank was ideal. I like the empty black look. Except, I don't like the glare or reflection on bottom. So, I'm running a black back with a 1/4" of fine black gravel on bottom to give a bare look minus the glare. Bare tanks are also very easy to keep looking good compared to planted tanks which require trimming and a lot of maintainence. However, there are problems with bare tanks. Unlike a planted tank you dont have all the biological filtration like plants and or all the surface area for the beneficial bacteria that help complete the nitrogen cycle. A bare tank with a large bio-load can be very unstable. I wanted my setup to have that bare tank look but still have all the benefits of a planted tank. Here is the theory on how I hope to achieve this with my setup. First, In the sump are 2000 1" bio balls with approx. 191 square feet of surface area. I have also made a drip plate to make sure all the bio balls are bieng used.(see bio ball section) This should support more than a enough nitrifying bacteria. Second, the sump also contains a freshwater planted refugium. A good grow light, CO2 injection, and plenty of plants should cancel out any nitrate build up from the bio balls and also help complete the whole nitrogen cycle. This should simulate a well balanced planted tank, while still giving me that bare tank look.