Hi Laurence and welcome. I think your main problem has been you have added too many fish too quickly. The tank would not have begun to cycle until you added the first 6 fish. Those products alone would not have done a lot to get it going. I like to work by the rule - add as few products as possible. Most are unnecessary unless you need to dechlorinate the water in which case you may leave the water sitting overnight and the chlorine dispels anyway then add it to the tank. Takes more time doing it that way though.
Undergravels don't usually cause big water flows so I am wondering if you have powerheads on the top rather than the water just bubbling straight up and out the top of the uplifts. They should have been fine.
Siamese Fighters, especially the males with the long flowing fins, don't do so well in a community tank and I prefer to see them on their own in a species setup. Having said that I am aware many people keep them
in community tanks.
You have added a total of 20 fish over a very short period and the filters could not keep producing more bacteria quickly enough to cope with the increased waste load. The last fish added are probably dying because they have possibly come from better conditions into your tank with climbing ammonia levels and they have been too stressed by the poor water quality to survive.
White spots on the fighter also point to poor water conditions and stress on the fish. This may be treated with a treatment bought at your lfs. The disease is called White Spot (for obvious reasons).
Have you done another water test since adding fish? What are you testing for? pH, ammonia, nitrite and nitrate readings would all be helpful.
Whatever you do, don't add any more fish!! Treat the ones you have, keep doing the water changes and see if things improve.