Many issues going wrong right now:
1.) You need to get your own liquid test kit. LFS
use strips which are inaccurate and often times they'll tell you what you want to hear so you'll get more fish.
2.) Stop adding fish. If there's a problem, stop making cannon fodder.
3.) You need to quarantine. You had a fungal infection that's normally due to very very poor water quality and other ailments. Fungus is often a secondary effect of other
4.) It sounds like you're stocking that 10 gallon pretty heavily. Add less fish.
5.) DON'T change out the filter cartridges. Just change out the carbon if you're running it and then rinse the cartridge free of gunk with tank water.
6.) Don't mix store water with your tank water. Temperature acclimate then dump the bag through a net then take a cup of tank water and rinse the fish out with tank water for a bit before putting them in QT or the DT if the tank is fallow. I personally drip acclimate all my fish/shrimp.
7.) Begin your cycle anew. Try fishless cycling we have a good article in the getting started section.
8.) Get a better filter. It's very hard to over filter.
9.) STOP getting more fish! If there's a problem fix it!
And for future reference:
1 gallon per inch rule is a horrible standard in such a small tank. Additionally instead of getting a menagerie of school fish get 5-6 of one type. These are just my opinions though so feel free to do as you will. The above suggestions 1-9 are musts though