Why do you say the Discus will be stressed from the Ph being low Rabbut? If they were tank raised in a similar Ph it will not cause problems, even if they where slowly acclimated to the Ph they would be fine too, either of could be very possible.
As for the bacteria dying from a Ph of 6.6 I have never heard of this either. Surely other Discus keepers have had their Ph lower then that and not had any problems with bacteria.
Most shop bought Discus are Asian bred and raised in a pH between 7 and 7.5. They have never seen the amazon, and it is unlikely that they have ever seen a pH below nutral. Even with careful aclimation, I have seen these guys stress. Work went through an obsession stage where the Discus had to be at pH 6.5. They were constantly stressed and picking up diseases. The boss quickly grew old of the issues with them and asked me for my opinion on them, as I was at the time the only member of staff who'd been able to keep Dicus alive for more than a few months. If a remady could not be found, they were going to stop stocking them. I surgested raising the pH by using just local tap water, and low and behold, all the problems went. I know a lot of people keep Discus at lower pH, and if they are German or Wild stock, it won't do any harm (it's actually needed for wilds as we both know) but the majority of Discus (in the UK at least) are Asian bred and appear to stress IME at a low pH
The Germans tend to use softer water with a lower pH.
Bacterial die-off for the most common pair of nitrifying bacteria starts arround 6.0-5.5. Hobbyist test kits aren't that accurate, so it is entirely possible that the pH in the OP's tank is arround this, especially if the kit is no longer "fresh". Below that kind of pH, a different set of filter bacteria form, that are slower growing. If the tank is cycled and the pH crashes, the origional set of filter bacteria die back over a few hours and then a new set of bacteria form in the filter over the course of several months
This explains why OTS only tends to show ammonia being present if the syndrome has set-in recently
ok just done a 50% water change and left it 15min. ph is 7, ammonia is just slightly higher than 0, and nitrate is 5
50% w/c in these situations are fine, as long as the temperature is the same, and the Ph doesn't move. Keep an eye on the Discus. The Ph just swung .4 which isn't really a good thing, a Ph spike can kill fish.
How often are you doing water changes? Are you using any peat or have driftwood in the tank?
50% waterchanges are fine so long as the temperature and hardness are similar. I almost always negate pH in acclimatisation and/or waterchanges. It is well documented in scientific litriture that a fish can addapt to a change of pH of upto 2 full points in less than 15 mins. pH shock is short-term and easily overcome. The true issue is a move in KH, as this affects how well a fish can offload ammonia and other toxins within itself (for freshwater fish). I truely believe that "pH shock" is actually hardness shock, mistakenly linked to pH because the hobbyist is not testing for hardness. I would be concerend about sudden changes of KH over changes of pH personally
pH is linked to hardness though, so low pH usualy means low KH and hence if you raise the pH you usualy raise the KH as well potentially hitting issues if done too fast. This said, going up a hardness gradiant is better than going down one
This also said, the importance of tempearture swings to healthy fish is questionable, as large temperature swings occur in the wlid...
As for the Krib, I doubt it is internal organ failure as Rabbut said. More likely he has some sort of internal blockage. Have you seen him pass a bowl movement recently? Try feeding him a green pea. Cooked, then de-shelled, and mash it up. If you like soak it with some blood worms in order to entice him into eating it if he doesn't accept it readily.
Bloating could be internal blockages, but with lots of frozen food going in, IMO it would be unusual to come across in this situation. DiscusLova's advice is worth a try though
tank itself has been running over 10years so if it's not cycled by now there is a problem. would I be right in saying over feeding would cause the ammonia though? (as i say chemistry isn't my strong point)
High ammonia could be caursed by overfeeding
This said, in a mature tank, raised ammonia from overfeeding will remady quickly as the filter bacteria multiply to consume the increased load.
water changes are daily, yesterday i done 50% twice, morning and night which i can continue doing if it will help.
currently there is no bogwood in the tank but there is a bit in the post.
and jim I know they wouldn't have been mothering it, they just looked it. haha they only look after there fry as long as there not hungry.
every breeder has told me they might not feed for the first week so i'm not too concerned at the moment as some are starting to eat.
however if I have a chemical inbalance will it not be my baby one who dies first? as he's swimming around happy.
ammonia and nitrate has just increased in the last few days so it's something i've done that has caused it. which is either removing the internal filter which is going back today, over feeding or water changes (maybe 50% a day isn't enough)
While ammonia is high, twice daily waterchanges are the best bet
It is not uncommon for Discus to refuse to eat for the first couple of weeks, but if they aren't eating the food you put in, you need to take it out before it can rot. I'd recomend only feeding enough to last a couple of minuites. Start adding little by little untill you can guage how much is actually being taken
In theory, changing chemical charactoristics will upset the larger fish first, as they have a larger bio-mass to change regulatory control over, but this may not the the case in practice...
For now, reduce the ammount you offer at each feed, but spread out more feeds across the day. This will reduce the ammount of food laying arround rotting and prevent the ammonia and nitrate rising
Adding back the internal would help the ammonia situation, but long-term I'd be considering another canister. Keep up with your waterchanges and the issue should clear away
Are your fish local bred stock? If so, I guess the breeders use local tap water? Local suppliers are great, as you can see the conditions they are in before purchase and ask all the needed questions about their care upto that point
All the best
Rabbut
Maybe the hatchery uses a single water holding tank and they chose to keep the Ph lower for breeding? Their water change or drip systems might be centralized in using this same water source too. Or it could just be their tap water 
