I will respond, not to be argumentative, but because there are less experienced members who read these threads and can take away the wrong impression if things are not fully explained. So with that in mind, I must comment on your first paragraph.
The fish are not yet fully grown.
The angelfish is about 4 inches and the pike and Bichir are about 6. I should point out that the pike is a Crenicichla Lucius which from what I've read, maxes out at around 6-8 inches. I agree with you that the tank is too small for them in the long term which is why I am upgrading to a larger tank, but in the meantime, they seem to be getting along perfectly fine, and I have no issues with aggression.
You cannot possibly know this, any more than I could. There doesn't have to be physical interaction to demonstrate aggression, it can be "silent." This brings us to your question to explain pheromones.
Fish communicate by chemical signals as well as visual and hearing. Fish release pheromones that can be read by other fish in that species. Allomones are released and read by other species. Fish use these to express aggression, submission, the presence of food, danger, spawning desire, etc. They are in the water and can only be removed by water changes. They are strong enough to be "read" in the habitat waters, so imagine their effect within the confines of an aquarium. It is possible to severely debilitate a fish to the point of death solely by means of these chemical signals. Think of it as putting a cat into a closed room in which a vicious dog is chained in one corner. The dog cannot reach the cat, but the cat doesn't know that, and will be terrified. So too these fish; they are severely weakened.
In terms of water changes, I am currently doing 2-4 20% changes per week. I find it hard to read the results of my liquid test kit, as I am partially colour blind, but I believe that nitrates are most often around 20ppm. Do you think this is adequate, or do you think I should be doing more?
I would increase significantly the volume of each change. I do a weekly change of 50-60% in all my tanks, and I would not do less than at least 50% here. I am assuming the parameters of the tap water are close to those of the tank water, so this should be no problem; you really cannot change too much water. [Parameters being GH, KH, pH and temperature.]
Nitrates should be as low as possible; these are toxic to fish, and cichlids are especially vulnerable. At 20 ppm, you are at the highest it should ever be, but is this consistent, or just before water changes? Some think water changes should be based on test results, so when nitrate increases, time for a WC. That is too late, the damage is already occurring. Water changes should keep nitrates very low, and stable water quality. My tanks run between 0 and 5 ppm, and have been this for years. I don't test nitrate very much, usually only if I see something not quite right, as an initial test. I check pH more often, though this too can be weeks apart, but for well over many months, the pH in each tank has never varied by more than a couple decimal places.
So just to clarify, I can reduce the flow of my filter with no ill effects on the tank?
I don't want to answer this "yes" as there are factors here that are the exception, or should be seen as such. I believe the fish species are non-compatible and thus the biological system is being impacted more, which in turn impacts the fish that much more. I would want to resolve this as soon as possible. Picking up an earlier point, you may think the fish are not out-growing the tank, but I would suggest they are do just that. Fish develop externally and internally for most of their lives, so the water volume as well as the physical space around them from fry on up is extremely critical. The external physical development can be separate from the internal organ development, and this is always debilitating, in extreme cases leading to stunting. The first impact of any of this is stress, and stress weakens the fish's immune system; 95% of all fish disease is directly due to stress. If the stress does not abate fairly quickly, it is irreversible. The fish will be more susceptible to disease, and will in all cases have a shorter lifespan.
Hope this helps, don't hesitate to question anything. B.