I agree with Ash. When I first started keeping fish I think that I spent at least $100 replacing dieing fish and buying snake oils that the "experts" at the LFS prescribed to fix all my problems (and didn't, of course). You can save a lot of money in this hobby by being patient, taking your time, reseaching, and learning before you take the next step forward. I only wish that I'd found this site before blowing the cash, not to mention the frustration at not being able to provide a healthy habitat...
With your stock and the age of your tank it's probably safe to assume, for now, that your tank isn't cycled and that your current health problems are due to ammonia. Assume the worst in the absence of knowledge. Do a 50% water change each day.
Get an ammonia test kit. Don't get the strips, get the liquid drop type, they're far better. Less convenient, but more reliable.
Remove the pleco and return him to the store. There's no pleco suited for a 10g tank, they all grow too large.
Feed your fish only every second day for now and in small quantities as Beeker suggests.
I'm mostly just reiterating Slappy's post...
FWIW about salt:
You do not need salt for healthy freshwater fish.
There's no functional difference between aquarium and table salt.
Advice on posting:
I know that you're new at this and don't know all the jargon, so don't take this as a rebuke, just as some advice on how to get better answers sooner.
Do a little research into your equiptment and inhabitants. In general this is something you should know anyway, don't add random fish to your tank. Don't be too shy to bring a pad and paper to the store to write down the fish names.
Start a thread with your tank specifics in it and link it to your signature (like I and several others have at the bottom of our posts). This provides those of us who are trying to help you with a clearer picture of your tank. Without it all we can do is guess at your problem. There are instructions on how to do so in the site's FAQ's or in Kasakato's signature.
Also, now that you have test kits, post the results quantitatively. Never use vague words like "fine", because we hear that a lot and it's often from people who later post that they have:
NH3=2ppm
NO2=1ppm
Or something of the like.