Well gang, enough was raised that I decided to first talk to the vet and get her opinion, thinking knowledge is power.
So her opinion is it isn't fish TB in the tank, because fish TB classically kills very slowly over months/years, not so quickly like here. She said we can still do the test, but that also she expects it would be positive because almost all fish and all tanks have it, but at a low level that wouldn't hurt the fish. I think she doesn't want me to be disappointed given that it is expensive, but I think it's good to try. So even if it is positive, we may still not know if that is the problem I guess. I'm not completely convinced, and I still want to do the FishTB test, but I will try her suggestions first anyway in case it does help. From what I've read fishTB can become more nasty, as it did with Diana Walstad. Maybe it is the case that fishTB scares online are all something else, and the fishTB people see as the most obvious problem even when it isn't. She also said that fish get fishTB from eating infected dead fish, which is possible in my tank, as I seem to have less tetra and pencils than I used to, I don't think I have found or euthanised all the sick fish even though I've tried to prevent this. I gave the fish so many hiding places thinking it would make them less stressed, but it also means there are places that they could die without me knowing.
I do suspect some secondary infections are going on and that's influencing the variety of symptoms though, such as the ich. The ich affected almost the whole tank very quickly, but it went away with a treatment of EshaExit. It came back a little, with 1 or 2 dots on maybe 3 fish that already had other symptoms and this was a sign for me to euthanise. I treated it again when I saw this as well and it hasn't come back in 2 weeks. Interestingly she confirmed to me that ich can come from adding new plants from infected tanks, which is one of my theories as to why my tank showed no ich for 5 weeks after adding new fish. I did add lots of new plants at that time. I personally am not 100% convinced that what is wrong with the tank came with the pencils+dicrossus, I think it's very possible other fish already had it in the tank as I have had a death here or there every month or two for the whole of the tanks duration, the pencils & dicrossus could've caught it while in the tank and are just extra sensitive to it.
So she's suggested I again treat with a higher dose of EshaExit, Esha2000, plus add EshaOptima to help the fish's immunity this week. And then once I've treat that then treat with FlukeSolve again to try and hit all the potential pathogens in the tank. She suspects some sort of protazoa primarily.
The vet's theory essentially isthat the fish are stressed for some reason in my tank and that's what has caused a lowering in the immunity in the fish and a few things to take over. After I described the tank and everything I do with it, she thinks that it could be because when I waterchange and add water back in - I just add Prime straight to the tank first - rather than pretreating the water (I don't have space to pretreat the water sadly, no bathtub). Another option she thought is that the fish could have been used to the higher pH of water at the London shop I bought them from (pencils & dicrossus), and that they were acclimating badly to the lower pH of my water. But then whatever they had has clearly spread to tetra and otos who have been in my water for a long time so idk. I think that overstocking is the most obvious potential cause of stress, but tbh it is puzzling to me that the pencils would show it as they were in a much smaller more stocked tank at the shop, and they have shown nothing but confident bold behaviour - it's not like they were hiding with off colours. Then again, I probably just don't understand fish stress well enough. Also she said that pencils are particularly sensitive too. The whole time my waterquality has looked good, no obvious problems there.
Another option for treatment is to do scrapes to test for parasites, and she said she could do it, or find a friend with a microscope and do it myself. This does intrigue me, anyone in London here have a microscope? I thought you would have to kill a fish to do this, but apparently it needs live fish and won't neccesarily hurt them so that could be good. My male apisto twice I have seen scratch his gills on wood since I got him, and I have seena fish or two over the past few months seem to itch (this is why I treated with FlukeSolve a while ago), it would be good to test him I think if it doesn't hurt him. Would be good to hear from people that do this, I know that koi people do, but has anyone done it with smaller fish?
On Friday I also made a video on my Instagram about my troubles (and that I thought it was probs fishTB). I got lots of lovely responses, and one that was particularly useful was an American guy who had a lot of experience at a fish shop and had dealt with confirmed fishTB before, and said his boss had dealt with it several times. They said they lost at least half the fish in their infected tanks. This is what is so confusing, that the vet doesn't think it's really a thing, but then other very experienced people do. Anyway, he said along with what I'm already doing, antiparasitic foods seem to help, they give frozen foods with Vita-chem, seamchem kanaplex, metroplex, focus & liquid garlic. Obviously can't do all of these because they aren't available here, but will try EshaOptima and garlic at least, and then maybe Seachem Vitality. And of course if it isn't TB, then all those things help. I have notices a few articles talk about vitamins as part of treating fish and getting them as healthy as possible to overcome disease.
I personally still have a hunch that it is TB, and that TB open a door to everything else that's lurking in a tank in small amounts to attack. But also LBR I have no previous experience with this stuff so what I think is not really reliable. I will try the above things and then do the test. I need 3 affected fish, and right now only one looks dodgy to me tbh.