Most of this has been covered, but I may have a bit more background for you to answer those questions in post #5.
Organics occur in any tank with fish. With live plants, most if not all of these can eventually be utilized. But without plants, these just accumulate. Algae is quick to take advantage, and there are many types of algae. Most are beneficial in the absence of plants because they perform some of the same task--using nutrients from organics and produce oxygen. But there are types that are not good, such as green water.
This is caused by unicellular algae that proliferates in the presence of nutrients (its food) and light (algae is photosynthetic). Photosynthetic simply means an organism that synthesizes carbohydrates from carbon dioxide and water using light as the energy source. The more light, the worse it will be.
Partial water changes and removing the organics in the substrate will help to prevent this, but with as much light as you have on this tank you are fighting a losing battle. First eliminate the direct sunlight; this is not good on any aquarium due to the heat aspect, but it also is very strong light. Reduce the duration of the tank light; as you have no plants, the light is solely for viewing the tank, so use a timer set to be on when you and whomever are normally home to view the aquarium. The fish will be perfectly happy with no overhead light at all, so limiting it is not going to harm them. Another aspect is the light intensity; again for solely viewing the tank, the light should not be bright, as this is stressful on fish and they will show their best colouration in less rather than more light.
Some floating plants which are easy to cultivate would be a good idea; they take up a lot of nutrients, and are close to the light so they shade the fish too.
Byron.
