Help! Sick guppy!

I have a 10 Gallon tank with 2 guppies, 2 zebra danios, and 1 cory catfish. I noticed yesterday evening that the guppy I have had for about a year now had what looked like a tiny white blister/pustule (no bigger than the head of a pin) on one side of his dorsal fin. On the other side there were two more that were smaller. The look like perfect globes that just got stuck onto his fin and is pulling it over onto one side of his body. He is swimming almost normally - but he is shimmying and resting at the bottom or on plants a bit more than he normally does. From looking at pictures and things I think it is a bacterial infection. So today I went to Petco and got Melafix which I have treated the aquarium with. Before I put the treatment in the tank - I briefly scooped both guppies out into a clear glass (instead of netting to cause less stress of course) and looked at him and now the underside of his belly is red and inflamed looking and it almost looks like it's eaten through his scales! It's terrible...but I treated the tank and am crossing my fingers that I'm not too late.

As I was trying to take a picture to post of my guppy - (I failed because it kept moving around) I noticed that one of my danios doesn't look right either! He swims so fast and never stays in one place it's almost barely noticeable, but he has a tiny white dot on one of his pelvic fins. In addition, both of his fins seem a little frayed. Could this possibly be fin rot? And if so - will the Melafix be good for this? It's crazy that they can look like this and be swimming so normally. Also should I be feeding them anything differently? My sick guppy kept going up to eat and then spitting out the food....

Also…after putting the aquarium salt in (I don't know if that makes a difference or not) I tested the water

Nitrate - High (160ish) Unsafe I know...
nitrite - Low (0) Safe
Hardness - High (150ish)
chlorine - Low (0) Safe
alkalinity - High (180ish) Ideal
pH - High (8ish) Alkaline

Yesterday after I noticed he was sick - I did about a 20% water change. I also added some API StressZyme (as well as API StressCoat as a conditioner). I don't know if these tests are serious enough to require an immediate water change again or if because of the new addition of the meds and the aquarium salt if I should just wait it out...

Does anyone have any thoughts on what this might be and if I need to be doing anything differently?