FAT HOMER is 100% right here.
You are overstocked and under-filtered. This guarantees sick/dead fish unless you change water about every day.
You should normally change water twice a week in this aquarium, when healthy, and basically just rinse the filters off in aged water and put them back. Re-use them as long as possible, under normal conditions. Frequent filter replacements kill bacteria, and bacteria are your real sanitation workers. They clean the water by reducing wastes to less poisonous things.
I agree that this looks like columnaris. This is a bacterial problem that I have treated with Kanaplex on my Jewel Tetras. The thing is that you can't wait until the fish looks "funny" over big patches of his body.
Even then, Kanaplex has saved some rather scabrous looking fish, with repeated treatments in a hospital tank, over 2 weeks and more. Some couldn't be saved because initially I was treating for fungus. In 45 years I had never had columnaris and misidentified it, because fungus can grow right away on skin damaged by the bacteria.
Nasty smells in the tank are normally decomposition of dead bacteria. Healthy bacteria make the water smell better, not worse.
Perhaps you put active healthy filters into a tank with a much lower pH, which instantly "burned" the bacteria and thus stunk up the tank.
I would divide up these fish if possible and treat the bad looking ones separately.
Remove any carbon/charcoal then do a 75% to 90% water change. Then dose with Kanaplex. Repeat this every two days. The 7th day, change water but no Kanaplex. on the 8th day, start the 6 day treatment again.
You will not get by with just one round of treatment, as I believe all your fish are heavily infected now.
After the 14th day, discontinue Kanaplex, and change water twice a week thereafter. Watch closely for re-infection.