I'm surprised that after five days of getting help from two very good Discus keepers, you are still treating the symptoms of your trouble, and not the cause. Your care is not up to scratch. This paragraph is the only time in this post where I'll be intentionally blunt. Now you are working on the symptoms, I'll try set you right on the cause. After my dig you'll need a moral boost, so I'll list what you are doing right before I cover what's wrong and how to fix it. Discus are easy enough to keep alive if you stick to a select few basic "rules" of keeping them. The fact you have had issues keeping Discus alive over any reasonable length of time should scream to you that something about your tank or care is not correct, and you need to work out what the issue is and correct it before you waste more cash and life on a tank that isn't working for what you are trying to keep.
Tank size. 55g is
just about big enough if it's a 55g tall (not long/wide)
You are listening to advice. This is always a good start
Now, a list of things I see as a problem (Grab a cup of Coffee, sit down, make sure you have 15mins spare and make yourself comfortable. Scroll down the see the list length and improvements I'd suggest to your care
)
Water. Well, where to start here... Cleanliness. Monthly water changes changes would struggle to keep up with the load on a lightly stocked tank, let alone a minimum size Discus tnak that is fed and stocked according to minimum guidelines. With adult fish you'll get away with 30-50% weekly as a minimum in a bare bottomed tank. With Juviniles or a substrate, you should be aiming to do 30-50% twice a week. Anything less, and your water will be loaded with Dissolved organics and (eventually) other pollutants/hormones, If you are raising juviniles in a tank with a substrate, I'd actually be aiming for daily water changes. If you cannot provide this amount of water changes, you have chosen the incorrect fish to keep. As with anything, water conditions are key to keeping these guys alive
If that level of water changing isn't possible for you, look at smaller and less messy fish that aren't as fussed about clean water
Next up, actual water parameters. pH and temperature are both too low. You are keeping (most likely as you have bought from a shop) Asian bred stock. They will not have seen a pH below 7.0 before in their lives if you ignore sipping when a CO2 build-up unavoidably lowers the bag pH water in the 30 something hours they are in transit for. The exception would be if you have a clueless LFS, which wouldn't surprise me. No Discus shop that knows what they are doing would sell Discus until all staff have satisfied themselves that they are healthy and all ave been seen eating. The fact that they sold one that wasn't eating would suggest to me a clueless shop. As covered above, your temperature needs to be 28 as a minimum for a Discus's immune system to be effective, with 30 being preferable (temperatures in centigrade).
Diet. You are feeding a very slim diet of high protien, fat and anti-vitamin foods. There isn't enough vitamins or variety there. Discus eat mainly vegetable matter in the wild, and aren't used to lots of protein and more specifically fat. You need to get more veg in there, ideally about 1/4, add a good staple pellet food (what do you have already?) and then increase your fresh or frozen food variety. Lots of Dicus keepers swear by Beefeart, but I will never feed it to a Discus under my care. My information surgests that it will severly shorten the live of Discus fish if fed as a staple diet. Breeders offering Beefheart to their Discus usually report 5-10 years live expectancy. Discus breeders not offering it at all surgest 25-30 years expected life span... As a treat of "grow out" food it may be OK, but it never wants to be offered neet, and NEVER as shop bought frozen stuff, the meat quality is poor. Make your own, and mix in other foods. The heart wants to be a maximum of 20% of the BH mix when you are done
On top of potentially shortening the life of Discus, BH is packed with anti-vitamins that cancel vitamins from other foods, lacks vitamins itself and is loaded with un-healthy fats and protiens that are supposed to be difficult to digest (I'll reference one of my books and pages for that info if you want/need it. While it is partially author opinion, the author does link to good and valid scientific research that appears to back-up his claims on BH nutritional values)
Filtration. If you were feeding the amount required (three feeds a day for adults, 4 for juviniles), one lonely hob rated at or slightly above your tank size would not cope with the waste loading on your tank. You want to at least double up the HOB with another that claims to be able to run the tank alone, or ideally, install a canister filter.
Group number have been covered already
Stocking. Tiger barbs are nippy and fast moving, and can easily stress Discus, I'd suggest their removal. BN's are fine, but what are the other Catfish? (sorry if you've mentioned this, I got fed up of all the things that hadn't been picked-up on by half way through the last page, and there was no way all the no-no type things could have been pointed out and explained in sufficient detain in the remainder of the posts I didn't read...)
Salt. Discus are supposed to be salt in-tollerant. While short-term IME this is not the case, using salt with ANY fish that isn't Brackish or Marine will do damage over the long-term. It leads to premature organ failure as a result of increased osmotic and toxic pressure on the fish. Water is constantly sucked out of the fish and the salt makes is harder for the fish to remove toxins from their bodies. Short-term this won't be too bad, in fact that's part of what makes salt good when treating White-spot with it, but long-term the increased strain on the gills and more importantly the Kidneys and Liver, will lead to organ failure and death.
*goes to read last few posts*
RE Kisuri Discus Wormer. It's as effective as asking the parasites to leave IME. IE it does not work. It doesn't even have the correct type of Flubendazole in it to work (without killing the fish, it will kill the parasites, but that type of Flubendazole requires a dose strong enough to kill the fish you are treating it kill parasites). It may slow the parasites reproduction rates suffice for an immune responce to overcome the ailment, but it is not effective for heavy infestations, only mild infections that can be shaken off by the fish anyway with good husbandry. You are in the US so sould have a large number of Flubendazole, Metro and Prazi meds available. Follow DiscusLova's advice on what to get, I can only recommend stuff that works to the UK folk, as we have to "improvise" to get a working Discus wormer with Reptile/Dog/Cat meds...
Gosh, I need to get a life, I've spent nearly an hour on that
All the best
Rabbut
Really, we all need to research for ourselves, the needs of the fish we are getting, and listen to the LFS only if they are usually on the correct track with advise. This lead onto answering the PM you sent me. The main book I refer to is Dieter Untergasser's guide to Discus Health. The book is fairly old, but most of the information is still correct and up-to-date IMO. On-line there is a wealth of information on Discus however 
