Julii Cory Terribly Ill

Hi Everyone!
New here, as well as to online forums, so forgive me if I don't really know how this all works. 
I've had a 10 gal aquarium since about March of this year and have had for the most of that time a male betta, 3 corydoras (julii, metae, paleatus) and as of a couple of months ago a Bolivian ram and a rainbow praecox, who I took from a friend's tank as they were being bullied.  I recently added an emerald cory.  I know it's pretty stocked, but I've been very vigilant about weekly water changes, and I love corys and couldn't help it...Oh, almost forgot, I have a lot of snails; it started with a stowaway who I just left alone and things seemed to be going swimmingly, until it got out of control.  Snails everywhere, along with slime coating the plants and wood.  I pulled out most of the plants except for the broader leafed ones which could be wiped off with a paper towel. The snail problem is getting under control, most winding up as dinner in my friend's cichlid tank, the conical ones hiding in the gravel during the day staying clear of the assassin snails I got to help take care of the problem...whew....ok
So my problem is that I noticed on Sunday after do a partial water change that the Julii cory was swimming oddly, in all directions and seemed to lack control then falling to the bottom and lying on her side with labored breathing.  She also has barely any barbels, pretty much none to speak of since I've had her, but has always seemd happy and healthy.  Beginning to wonder about the metae cory, who I don't see much of anymore since I made a little hollow beneath one of the rocks.  Is that normal??  The others swim like mad!

Anyway, Water params before change:  pH 7.4, amonia 0, nitrite 0, nitrate 40ppm (i've had an awful time with nitrates; stabilized after the nitrate spike after tank set up.  I think it started after I pulled out a lot of the plants) water temp 76F (check regularly)
Winken, the julii, has been just lying ever since, moving to different locations from time to time.   I think I made a hasty mistake, though, taking the advice from the manager at my local pet/fish store.  I go there regularly and know the manager and she seems to know a lot about fish, so I figured why not.  I was going to buy melafix at recommendation of a friend but I saw the erythromycin em which seemed to be useful in treating various infections.  After some hesitation, I put the packet of meds in the tank, hoping for the best, then started to wonder if I was jumping the gun.  I don't generally take antibiotics myself. 
Now I'm beginning to wonder if it was an illness that was caused by an infection induced by the eroded barbels???
 Have I compromised my tank by adding antibiotics needlessly, as there seems to be no improvement, and I'm basically killing off all the nitrifying bacteria and submitting the other fish to a course of antibiotics they might not need but has to be finished.....uugh.  Also, should I add even more sand to the aquarium?  Right now it's sand and the smallish round black rocks you get at aquarium shops.  The others' barbels seem to be fine, though I need to check the metae to be certain.

Oh, my.  I'm sorry to have babbled soo much.  Any advice would be much appreciated.
Thanks!!!
Stephen

Have you seen any problems with any of your other fish?? If the rest of your stock is fine it may just be that this sick fish was naturally weak or could be seeing the effects of old age or has had an injury.

Personally i wouldn't add any chemicals to my tank unless i was sure i had an underlying problem and was 100% certain i knew what it was.

I can understand why you like corrys they look great together huddled on the bottom of a tank.  your assasins will get there in the end with your snails just give them time but definately take note of sues advice on feeding.