Problem with silica sand is the fact you need to wash and wash and wash umtil you are left with approx %50 to get rid of the finer dust particles. I now this from having done this with over 20 5Kg bags of the stuff. Also it is very dificult to keep clean despite people claiming their fish do the job for you.
Severum and barbs originate from water with a ph of 6.5 bristlenose and kribs ph 7 most tap water is around 7.2 dropping to below 7 ( these fish would be fine together ) Most African cichlids prefer hard alkaline water with a ph above 8 for best results.
There is a stream on the Napier-Taupo highway which feeds into lake Taupo where years and years and years ago someone introduced swordtails, parts of this stream are very overgrown. I have been to this stream but found nothing.
Fully grown discus need between 30 /40 litres each. I suggest you contact Discusguru again he really IS an expert. But to keep so many in such a small amount of water is asking for trouble. You will need to do excessive amounts of water changes and check water parameters on a very regular basis. Also your opening line states that it is a 100L 60x45x45 sized tank.
The red test liquid is supposed to turn green but did not on any of my tanks or tap water or rainwater it just became a deeper and deeper red. I do regular water changes and I am fairly sure that tap water does not come out at over 2,000ppm.