Was the tank dry or without fish for a time? Unfortunately that would mean that virtually all beneficial bacteria would have dried out or starved. That is something which your fish seller could have and should have told you. They, as much as you, are responsible for you adding too many fish. Also when you draw from a couple of shops, without quarantining the fish, I fear that you dramatically increase the chance of diseases being introduced.

Was that white spot what is often called Ich? What did you treat it with? I'm glad that it went away. If it was Ich, you need to continue the treatment for 9-14 days. (See the cycle of the stuff in IH.)

Also, you can do your fish a huge favor with partial water changes of 25%+ at least once a week, more if possible. Do treat your water with whatever water conditioner your LFS (Live Fish Store) suggests will deal with all of the harmful stuff put in by the water department. Let the water sit, top open, for a day or more if possible (seasoning water). By gravel vacuuming a part of the tank before each water change you also get a lot of the Ich organisms in the gravel out of the tank. If you take out 25% of the water, add 25% of a treatment dose with the new water.

I ran over your post and did a cosmetic proofing. "have added filer start" stumped me. Is this an additive of beneficial bacteria? That is a terrific idea if so. Follow the instructions as faithfully as you can. Please, when the crisis has calmed, tell us more about the product.

Kudos for testing and reading as much of IH as you can.  I realize it all can be pretty overwhelming. I have to re-refer to it from time to time too.

The numbers from your tests don't sound too bad and that is a little puzzling. Ich sometimes results from the fish being chilled. 24C isn't too bad, but while treating for Ich, can you walk it up a degree a day for three days?

J-ME may be right about there being a bacterial problem. The frequent partial water changes, with treated water of the same temperature as that of the aquarium will remove stuff the bacteria is living upon, will remove some of the bacteria and may enable the fish's immune systems to more effectively combat the bacteria. The more partial water changes, if done right, the better a chance for survival the fish will have.

If there are no bacterial outbreaks, it could just be Ich, the water changes, done right, will still be beneficial.

Be really careful to not over feed. Everything should be consumed in a just couple of minutes.

A lot of people suggest belting the tank with an antibiotic for bacterial things. Putting the fish in a hospital tank for that might be better. Medicating the regular aquarium with an antibiotic will kill most all of the beneficial bacteria which break down the ammonia and nitrite. That leaves the fish swimming in a broth of toxic wastes and probably kills even more fish.

You are one the right track in several ways. Hang in there!