You (Woody781) have raised a great many issues in the successive posts, and it would take any of us a book to explain all of it. I will highlight a few things as background for moving forward. [And speaking of background, rather than paint, a cheaper alternative is a piece of plain black construction paper taped to the tank frame; I use this on my smaller tanks, and it increases the perception of space in the tank, making the back wall almost disappear, and fish and plants look lovely against black. Make sure it is not shiny--just get a sheet of black construction paper from a hobby-type store.]
Each species of fish has specific requirements it needs to be healthy. These requirements are inherent to the fish, conveyed by its DNA. So a fish "expects" certain things, and if one or more of these are not present, it will be under stress. Stress is serious, because 95% of all fish disease is directly caused by stress that becomes too much for the fish's physiology to handle. So we must aim to reduce and even eliminate as much stress as possible. The fish's requirements are the first step.
Water parameters are very important; these are the GH (general or total hardness, which is the lewvel of dissolved minerals in water), KH (carbonate hardness or Alkalinity) and pH (this is how we determine if water is on the acidic side or basic). [I cannot detail all of this, it would really take a book, but I just want you to be aware of things we need to look at with fish in an aquarium.] You should be able to ascertain the parameters (GH, KH, pH) of your tap water from the water authority; check their website.
Temperature is also a parameter of water, but one we can more easily control with aquarium heaters. Temperature drives the fish's metabolism, so it is critical that the fish be kept within a specific range for which it was designed by nature, otherwise it will be under stress and not function properly.
The aquascape is also very important; the habitat of the fish in nature is the key to what it needs. The substrate (some fish must have sand, some are fine with fine gravel, some may need dried leaves), and hardscape like wood, rock, or artificial decor to represent these, maybe plants, is vital. The overhead lighting affects fish. The current produced by the filter affects fish; some need more, some less, some none at all. Your neons like it dark, so dim overhead light, or floating plants, provide this; they also like slow-flowing water, contrary to the Borneo catfish that must have a current. Unless the aquarium is very large, say 4-5 feet in length, it is not possible to provide both, so one of these species will be at a loss.
With all the above factors in mind, we come to a community tank, which means an aquarium containing more than one species of fish. Here we have to consider the above requirements for each intended fish, and make sure that each species we intend has similar if not identical requirements as the others. Because these needs are programmed into the fish's DNA, they cannot be ignored because they are vital to the fish's well-being. Deprived of any of these, the fish will be weakened, under stress, more likely of disease, and inevitably a premature death.
The fish's behaviour also comes into this; naturally aggressive fish cannot be combined with quiet sedate fish.
Byron.