Probably the single most important factor in keeping Discus is water and its quality. There is lots of information out there but for the beginner I think it can be confusing and a worrying factor in Discus keeping, it need not be honest.
The single most important factor (if the tank is mature) is PH as long as the pH is on the acid side of neutral, between pH 6.0 and pH 7.0, your Discus will be quite happy.
Stability of Ph because of 'soft' water can be an issue for the beginner and as such until a little is known about the interactions of calcium hardness(Kh) / General hardness (Gh) on the acidity of the water(Ph) staying with water of 12 to 15 Gh and 5 to 7 Kh should give you stability of Ph and happy discus.
Nitrate, Nitrite & Ammonia
Ammonia (1st stage of the nitrogen cycle) is a killer this is the waste products from your fish and they are swimming in there own excretions. These must be maintained at 0 at all times or your fish will suffer and probably die.
Keep an eye on these levels when adding new fish to your aquarium whilst the biological capacity of the filter catches up with the increased load.
Nitrite (2nd stage of the Nitrogen Cycle) is a killer and levels should also be kept as low as possible. This can be achieve by frequent partial water changes and or encouraging good plant growth.
Nitrate (3rd stage of the nitrogen cycle) levels should be kept as low as possible (below 25ppm ideally) but is not as toxic as Nitrite and ammonia and won't kill your fish overnight & quite high levels (above 100ppm)
A heavy metal filter, (sometimes called metalex) which consists of a pre filter pod, carbon block pod and a CBR2 pod will remove almost all compounds that will harm your Discus fish. It will not however remove hardness.
If you live in an area with tap water which as a general hardness (Gh) of over twelve degrees, a reverse osm
Both Metalex and RO units will require periodic replacement of the cartridges. The frequency of cartridge replacement depends on the levels of contaminants being removed from the tap water and the volume of the water being processed.
If algae is or becomes a problem it's worthwhile testing for phosphate which can be introduces in the food you feed fish. High phosphate levels can again be reduced with partial water changes which are by far the best method as you will remove other things like nitrate as well.
You may also use a proprietary media like Rowaphos, placed in the filter this will remove the excess.