Ok doing a post recording the various deaths of fish and what it looked like over the past 2 weeks. This also started about a month after I bought the pencils & dicrossus. I don't have photos of all the fish, and looking back I haven't euthanised the fish all immediately when I saw a problem, I have tried to wait a few days to see if it improves but tbh it never does (except for ich). Very grim tbh.
Death 1 18/02/2022: this pencilfish blew up like a balloon for a few days (when I first saw it it was smaller and I assumed a female with eggs), who I took out to see if it responded to salts and then realised was pineconed and had dropsy. This was a good week before everything else started, I didn't suspect anything at that point.

Death 2 & 3 (not sure which date this was): I didn't photograph the second one, but I saw one of my big older tetra with a white "graze" on it, and thought hmmmm, columnaris? 2 days later I saw it in the back, the graze was worse (more 3D/fuzzy) and it wasn't swimming around so I caught it. I got the 10 tetra about 10 months ago, one has had really short nipped fins that never grew back the whole time, and I noticed this one also had a few dodgy white scales so even though it was still swimming around, it looked vulnerable to me and I euthanised them both. I thought at this point that these tetra were probably just getting old and that was the problem.

At this point the tank got ich which I treated with Esha Exit. I didn't have any deaths from the ich and it went away quickly. Only tetra got the ich on the body, to rest just on fins before going away.
And then I noticed a dicrossus had a few tiny pits in the head, and thought... uh oh.

Death 4 & 5: These fish were hanging at the top of the tank together - an anorexic pencilfish that was bent, and a bent fat old tetra with another white graze. I didn't take a pic, just netted them out and euthanised. This was last week, and about when I made the post about how I am suspicious and suspect the fish tb/mycobacteria. Sadly didn't take a pic.
Deaths 6. 7, & 8 12/03/2022: I noticed 3 dicrossus now had these pits and they were getting worse, so I took them out and put then in a quarantine tank. 2 died by the next day, and the third I euthanised as the pits looked bad and I realised they coudn't go back in the main tank like that. This was last week, can't remember the exact date. This is the worst one, it's hard to tell but the head at the top looks like it's eaten away. Honestly this was the ahrdest one, they are such lovely fish.
The morning of this day I woke up to find a pencilfish dying upsidedown on the floor of the tnak, to took it out and euthanised it. This pencilfish had a similar graze on it to the first tetra pictured for 2 days before, but had been acting fine.

Death 9 13/03/2022: I had noticed a few days before a pygmy cory with its fins eaten away, and knew I should kill it but didn't have the heart yet. But I knew I really should be euthanising them when I see them and just did it. When I caught it I realised it had ich, I didn't take a pic though it was at night right before I went to bed when I saw it. This is the only pygmy cory I've seen with an illness so far, though I suspect a few more have died since I added them. Although none of the other fish are showing it, this little pygmy cory seemed to have ich so I treated the tank again. This seems to be a telltale sign of the near end of a dodgy fish, the ich comes back on them only.
Today - Deaths 10 & 11 16/03/2022: Another anorexic pencilfish who I had noticed for 2 days and hasn't improved, along with a pencilfish who I hadn't noticed with problems before but today had a really gross injury - the gill on one side was exposed like it's face had been eaten away with scales hanging off (hard to tell in the pic how bad it was). I think maybe before this pencilfish had a paler head for a few days before, but that's it. The pencilfish seem the most prone to this illness so far, I've euthanised 5 of the group of 15. I have also noticed at times some pencilfish have stringy poop, maybe 2-3 times but it goes away after a few hours, I have euthanised those fish, though maybe I should on reflection.

Another option is that my tank has columnaris (the dodgy looking tetra photographed above looked like this) and hexamita parasites (which causes hole in the head in cichlids), which could be a possibility, but with the breadth of symptoms across many types of fish I think the TB is most likely.
Here is a list of symptoms of fish tb/Mycobacterium from
<this useful video> &
<this link>, I have bolded the symptoms I've seen with my fish in the tank.
Emaciation
Not eating
Reduced acitivity levels
Loss of buoancy/seem heavy on the floor of the tank
Dropsy
Bent spines/deformities
Lack of colouring
Redness on the skin
Skin lesions
White bulging areas under the skin
Pock marks
Loss of scales
Fin erosion
Popeye
White faeces
Areas like mouth rotting away
Mouth ulcers (can look like tumours, usually on bigger fish)
Redness around vent
Swimming haphazardly/spinning
So far I've been feeding the fish well with live food, continuing my 50% weekly waterchanges with an extra 30% one mid-week, a big gravel vac, filter & pipe cleanings. I've also bought a massive UV steriliser - the TMC vecton 600 which I will install tomorrow, and I hope will save some of the fish, or at the very least will save me from catching it getting a nasty rash and save the plants/tank. TBH After being quoted I don't think I can really afford the vet right now on top of the steriliser (which at least will treat things a bit), so will have to wait a while to save up for that.