Finally, my Community Tank is stable. TLDR: The most important parameters are Temperature > Ammonia > PH > TDS.
When I reported my fish were dying in my new tank, some people here did give a lot of aggressive comments about removing all fish, cycle tank, replacing all water with RO water, and whatnot.
The trick was maintaining a stable temperature. I am using two 200 watt heaters to hold a temperature around 25 degrees centigrade. Two heaters for redundancy, i.e., in case one fails, the other can work. It is the cheaper option than buying one expensive heater, which costs 20 times more than one cheap heater.
All the Ammonia, Nitrite, and Nitrate levels are coming to zero or below 0.25 ppm. I did buy the External Canister filter, but it was set up poorly with cheap filter media, like ceramic rings, sponge, and activated charcoal. So I replaced all the filter media with those off-white yellowish balls with many pores type of bio media. I still have one layer of activated charcoal in the filter.
All the other hype about 0 TDS 6.5 PH is unimportant for my tank setup. Most livebearers like Mollies, Sword Tails, Platies, etc., do just fine with 8.0 PH. The other fishes like Dwarf Gouramis, Rainbow Sharks, Barbs, and Tetras also don't mind the 8.0 PH either. The best way to lower the PH is by adding Driftwoods. The tannin from the driftwood made my water slightly yellow, but it did drop the PH to 7.8 and stay stable there. I have also added two of those Indian Almond Leaves to the tank, but I am unsure if they are doing anything.
People made a lot of fuss about TDS, for which I made two water changes of 100 liters of RO canned water each week. After the water change, the TDS was 250, but now it has risen to 300.
I realized that the best way to control the nitrates or reduce the needed water changes is to add plants to the aquarium. Since my community of the medium size fish will devour those fancy tissue cultured plants, it's not an option for me to do a planted tank setup. Instead, I found a better and cheaper solution, i.e., use the plants that have their roots in water but leave them outside. Money Plants to the rescue. I have many money plants tied to the tank upper rim so that only their roots stay in the water. I also added some cheap water cabbages. They just float around on the water, sucking up all the nitrates.
EDIT: After another Year into the Hobby, Temperature is not so important, if the room temperature does not change often. Plants are must and the easiest way to handle the ammonia issues. Bt more on them in a different post.