Well, no tank cloning or fishless cycling going on will be ringing the alarm bells of any experineced aquarists.... Let me explain;
Ammonia cycle;
Ammonia is produced by fish. This is broken now into nitrites and then nitrates by bacteria in the filter. These bacteria are
not already there, so you need to build this colonie up. Great, I'll add a few hardy fish, like the LFS advised to start the cycle. Yes, it will start the cycle, but;
it intentionaly exposes the fish to leathal ammonia and nitrite that will permenantly damage the fishes gills.
If the fish are lucky enough to survive this, they will live shortened lives
OK, fine, I'll add ne of those bacteria in the bottle products, that the LFS advises.
you could, but only two work. Bactinetts, in part and biospira, both of which have to be kept refrigerated at all times to work. If the product isn't refrigerated, it will have the normal metbolic rates going on within the bottle. This means that a hermetically sealed bottle containing bacteria would run out of air and die in hours. No bacteira. Is stability refrigerated? no, then the bottle is a useless money making ploy.
so how do I set up the tank?
You can either use ammonia to fishless cycle, as per the pinned thread at the top of the forum, or use mature media from an established tank.
where does this leave me?
Take a look at what your water stats for ammonia, nitrite, nitrate are doing, and I'll bet the first two aren't on zero, where they should be.

You will need to test ammonia and nitrite and nitrate when you get discus, take it from me, a discus keeper myself, that ammonia and nitrite at even trace levels are worse for discus than an off pH
The knife fish and silver dollars aren't compartible with discus, so if you want discus later, you will have to re-home them elsewhere. Knife fish will get over 3 foot in length, so plan that into your future plans for the tank
Prime is a good water conditioner, and may be the reason as to why you haven't yet had any deaths. It converts ammonia to ammonium, thus making it less toxic. Just wait for the nitrite to spike, and you will see a different story
The clown knife fish circling each other is actualy fishting. You need to get one out before the weakest gets killed

A lone discus will refuse to eat and die, so get a group of 5+ to "test the water" so to speak when you decide to get them. Discus have a reputation for being hard to keep because few people bother to research them before buying. If you don't know about their specific care before purchase, you will be watching within weeks as you £200+ investment flakes over and dies
Waterchanges for discus usualy need about 50% weekly or two 30%s a week or 10-20% daily
All the best
Rabbut