Other essentails
Test Kits
Cannot stress the importance of testing. You cannot tell by looking what you water is like. Test regularly and test accurately. It can be useful to log measurement to see how they change over time but at least once a week you should be checking you pH, Ammonia, Nitrite and Nitrate.
Some kits are easier to use than other and some seem to be more accurate but find one you like and make sure you use it. The chemical kits are better than the quick dip strips in my experience.
Air Pumps
Aeration is very important. A stagnant puddle is clearly not as healthy as a bubbling stream. Aeration has a number of effects, if you want to get very scientific just search Aquarium Redox.
On a more basic level aerating your water will help your filters (the bacteria in there need oxygenated water too) it will also help your fish. You will soon know if there is low oxygen saturation in your tank as the fish will be visibly stressed and breathing rapidly. The higher the water temperature the less oxygen it will hold. Therefore at Discus temps all tanks should have some form of aeration or you need to get the surface agitation from the filter returns to a suitable level to maintain the oxygen saturation.
Thermometer
Temperature is essential and it needs to be accurate and constant (see heating). It must be between 28 and 30 and you need to know what it is. A surgical thermometer is an excellent way to make sure you can measure the temperature accurately.
Nets
You will need to move fish or remove dead fish so this is an essential
Medicines
Always good to have these ready to go rather than watch an ill fish deteriorate while you go shopping or wait for a delivery. Always have some Octozin and Hexamita to hand and some Anti-fluke or wormer ready to go. Epsom salt is useful too but hopefully any emergencies can be avoided by proper care in the first place.
TDS Meter
Total Dissolved Solids is a good indicator of how good you water is. A TDS meter can be dipped in the water for an instant assessment of your water. Tap water gives a fairly high reading, RO (Reverse Osmosis) water should give a zero reading or thereabouts. If you want to breed fish you could try and keep this low. Most commercially available but non wild Discus seem happy in tap water (with a conditioner) so this may not be really necessary.
A Hospital/Quarantine Tank
This is the most useful thing I have found to make keeping these fish much easier. If you can have another smaller tank running all the time definitely set one up. Any fish issues in the main tank, you can move the fish and treat accordingly straight away. This is also essential for any new fish you buy as you MUST quarantine any new fish before adding to your main tank.
In a separate tank you can treat a fish, remove the stress if its being bullied and make sure it can get to food easily.
One of the main issues I see in the larger tank is the larger fish do better as they get to the food first. The smaller less confident fish start to eat less, then they dont eat, then they hide and over a few weeks they decline quite quickly. The moment a fish stops enthusiastically feeding in the main tank they are moved into this tank where they soon start to feed and grow again. This is down to the higher temperature and the ease of access to the food.
On a smaller tank like this its very easy to do daily large water changes, they take a few minutes. This does seem to speed up the growth of a fish. Plus this runs at 32 degrees which speeds up their metabolism and makes them keener to take food, which is easier for them to get due to less competition.
So highly recommended if you can do, once the fish is happy again and growing they are moved back to the larger tank.