Remineralize RO Water by adding Additives or Mixing other Source of Water with RO

Sharing my whole bad experience (lengthy description).

Since January 2018, I had been using a three-stage filter (01 Micron Sediment and 02 Carbon Filter) to treat Tap water for my Discus Fish (14 pcs at that time) Tank. My fishes were healthy at that time. I used to do 50% water change every 3-4 days. But last November (Winter season), my fish started to get sick. Then I checked the water parameter and found horrible result.

Parameter of Main Tank Water on November and December’ 2018)
TDS- 400+
PH- 8.2
NH3- 0.5 to 0.75
NO2- 0.25
NO3- 100 +

To overcome the problem, I have been using 02 days aged pure RO water since January 01 of this year.
RO water parameter was:

Water Source- Aged RO Water:
TDS- 20-30
PH- 6.4
NH3- 0.25-0.50
NO2- 0.25
NO3- 5 to 10

But I was struggling with the Nitrate level. It raised to 100 ppm(+), just after 3-4 days of water change. Then I have reduced my fish stock from 14 to 9. But my fish was not happy and the growth rate, color was not satisfactory (All of a sudden; move very fast Rub body with objects; Rub vent with object; Twitch Ventral fins quickly for a few seconds; Vibrate dorsal, anal and pectoral fins; white pimples on body). Then to solve the Nitrate problem, I shifted my fishes and Plants (Bogs with Anubias plant) to Hospital tank and treated full tank (February 15, 2019) (Main and Sump) keeping the biological media with Potassium Permanganate and Hydrogen Peroxide (Considering Media might be clogged or exhausted).

Now the Nitrate level raises 40 ppm after 4 days of water change and fish are happier than before but still one fish is quite sick and growth rate is not good. I think this is because of my usage of pure RO water as it doesn’t contain beneficial minerals. To control my quick Nitrate raising (40 ppm after 3-4 days of water change), I have already ordered Sera Seporax Biological Media and already bought Commercial soil to put some Low-Tech Plants on the Main tank (Soil will cover 1/3 of the tank).

Required Assistance (Considering Minerals for Fish and Plants & High Nitrate Problem):
Option 01: What Additives should I mix with Pure RO water (please provide low cost DIY solution)?
Option 02: Ratio of mixing of other Source of water (Tap water/ 3 Stage Filtered Water/ RO Waste Water) with Pure RO water.

Water Parameter (tested by API Masterkit) of all Sources:

Water Source- Instant RO Water:
TDS- 2 hrs aged-23, Instant- 51,71,129 (Checked Multiple Times)
PH- 6.8
NH3- 0.5 to 0.75
NO2- 0.25
NO3- 10-20, 10 (Checked Multiple Times)

Water Source- Aged RO Water (2 days):
TDS- 17,19 (Checked Multiple Times)
PH- 6.6
NH3- 0.5
NO2- 0.25
NO3- 5 Plus

Water Source- Waste RO Water
TDS- 550
PH- 7.4, 7.6 (Checked Multiple Times)
NH3- 4 to 8
NO2- 0.25
NO3- 5, 5 (Checked Multiple Times)

Water Source- 3 Staged Filter Water
TDS- 496, 500 (Checked Multiple Times)
PH- 7.4
NH3- 4 to 8
NO2- 0.25
NO3- 5 Plus

Water Source- Tap Water
TDS- 488, 527, 509 (Checked Multiple Times)
PH- 7.4
NH3- 4 to 8
NO2- 0.25
NO3- 20 to 40

Water Source- Main Tank Water (Last Water change 4 days ago)
TDS- 47,46 (Checked Multiple Times)
PH- 7.6
NH3- 0
NO2- 0
NO3- 20 to 40

Tank Details:
Main Tank- 48”*24”*24” (Bare Bottom) ; Sump Tank- 33”*15”*17” (3 chambers);
3 Bogs with Anubias;
Fish- Discus 09 pcs (6 are adult), Bristole Nose Pleco 09 pcs, Rummy Nose 22 pcs, Corydorus 06 pcs, kohli Loach 03 pcs
Food Habits- Usually I feed them 03 times. Tetra Flakes, HIkari Bio Gold, SanFransisco Bay Dried Blood Worm.

4-8 ppm NH3 in your source water!?

You could establish a biological filter to age your tap water and process the ammonia to nitrate. The ammonia levels might actually be too high to enable a biological filter to establish; for a fishless cycle it is typical to keep the ammonia below 5 to not stall the cycle. While high nitrates aren't ideal they are certainly better than high ammonia.

The alternative is to switch to straight R/O water, you can reconstitute with commercial products like RO right and discus trace or if you are comfortable with chemistry, some combination of baking soda, epsom salts, potassium sulfate other commonly available dry ferts for aquascaping which should get you to an appropriate GH, KH and pH.

Yes, unfortunately my Tap water has 4-8 ppm NH3 leaving me only option to use R/O water. Considering the less amount of minerals in R/O water, please advice me the combination of commonly available chemicals.

Thanks in advance

You are best to find a RO remineralizer like RO Right or Seachem Equilibrium (GH only, sulfate salts) Seachem's buffers (acid, discus, alkaline and neutral) are phosphate based which can lead to an algae bloom.

I'm not comfortable with recommending a mixture of commonly available chemicals without experimenting myself and coming up with a reasonable solution. greenleafaquariums.com has a good store of what could be purchased in the USA. A combination of GH booster, monopotassium phosphate, and baking soda would be where I would start.

If you can age a mixture of RO and tap (say 1:1) with an established filter you should be able to convert your ammonia to nitrate to manageable levels but it'll never but in the <10ppm range many shoot for.