Thank you for the reply Caryl.
I have a testing kit, parameters are all fine - no ammonia, no nitrite, nitrates are on about 10ppm but I add fertiliser once a week and have been assured this is ok. In terms of pH I’ve never been able to get an accurate reading - it’s always at the highest number of the regular pH check and at the lowest number of the high pH check, so I think it’s somewhere between 7.4 and 7.6. The tank is quite heavily planted but has a gravel substrate.
The thing for me is that all the fish getting sick are the oldest ones that I got when I inherited the tank. The only issue I can see is that for the two recent ones they are in groups well under what the recommended amount is - I had 3 X-Ray tetras, one died randomly and now this one of the last two is having issues. With the barb we had two very mature ones, one was killed by the pleco (which has been rehomed) and then the other one got sick and I put her down on Friday. I had bought 5 more barbs to bring up the numbers but they were very juvenile and they stayed separate from the older barb.
I mean it might just be legacy health issues for the fish that I inherited from the old owner - I’ve had the tank for a year and the more I’ve learned, the more I’ve realised that the previous owner wasn’t good at looking after the tank.
The X-Ray is twitching a lot today and seems to be uncomfortable, so I’m going to give it an Epsom salt bath, but outside of that I don’t really know what else to do.