Dominic welcome to the Forum!
I'm happy to hear you will be trying Discus and very thrilled that you are starting it right by asking first , then doing it! It will make your experience much better!
To answer your question,
Will the current stocking ideea be viable in this system (100 rummynose + 11 discus) ? in a 125 gal.. Will be crowded. The discus numbers are fine, but you would probably be better off with a smaller school of tetras..20-40. 100 will be too much I fear , especially at feeding time. Thats just an opinion. I currently have 35 discus at 4" in 125 gal as a grow out tank. Biologically your tank can handle the numbers, but aesthetically it may not work you. Thats a personal call.
. Will the floating plants and the ones in the water be enough for removing excess no3 ? i have a big reactor that is waiting to get wet in case i need to make something like a nitrate tower.with that kind of water change probably not enough plants, but you are buying adult discus so won't need to feed heavily , which will help alot. This one is one that will vary also depending on what you feed and how much.will plan on doing weekly 30 or 50% water chantge in the display tank.
This mix would be great in a bare bottom tank with lots of water changes, but in your tank, it will probably foul the water and bump up ammonia products.he food will be a home made recipe made of:
Beefhart
Salmon
Sardines
Shrimp
Brocolli heads
Spinach
Carrots
Garlic
Peas
Spirulina powder
Vitamins
Krill powder
(maybe noori sheets)
The krill and spirulina powder will be used as binders.
I would like to point out a potential weakness in your plan. I would not add the tetras first . Get your Discus first, get them all at once from the same vendor... Let them settle in and take the time to learn about them and get the tank balanced. Add no other fish, not even dither fish...Once you are sure your DIscus are healthy and happy then get your other fish and quarantine them separately for 6 weeks minimum.
Hth,
al