Even with a state of the art filtration system you can have problems with high nitrates in a predator tank. Don-in-SA has one of the best filtration systems I have ever seen and I know he has had problems with nitrates in the past. Predators simply produce a lot of waste and a lot of waste equals a lot of nitrates. Its a simple matter of your bioload exceeding the capability of your system to handle the load. There are two solutions: 1. Decreasing the bioload, 2. Increasing the capability of your system to handle the load.

Its not a matter of how many fish you have, but the size and type of fish you have. Two equal sized fish can produce totally different amounts of waste. For example, a tand and a triggerfish. The trigerfish is going to produce more waste because it survives on a higher protein based diet. High protein foods such as frozen meats contain a lot more protein then something like spirulina flake which has more fiber. Protein is the source of most of the nitrates because they are high nitrogen compounds.