Ammonia poisoning is the most likely cause. Even though you are not seeing ammonia i would still do plenty water changes. If nothing else it will maintain pristine water conditions which will help whatever the cause. Even if you are not seeing ammonia now, it could have been caused by an ammonia spike before you noticed the redness and the ammonia is now gone. Clean water is the only solution for ammonia poisoning i know of.
Parasites and bacterial infection are other less likely causes. Im going to make a presumption based on the medication you have tried. You are UK based? Unfortunately here the medication available in the US is vets prescription only here. The Waterlife products you used are about as good as we get over the counter here, and myxazin is what I would recommend. You could try esha2000 as an alternative. Interpet brands as a last resort, imo
interpet is kill or cure, its pretty crude.