As your post relates to 'they', I presume there is more than one fish and they all show the same symptom.
It very much looks like they suffer from some sort of infection that affects their swim bladder. Go to an aquarium shop (not a general pet shop) and take a sample of your tap water and tank water. Most would test it for you and suggest a possible solution - although don't expect miracles. By the time that you can see a fish is not well, it is often too late.
The arch you added may not be designed for a tank and be painted with a water based paint or one of the modern 'natural' paints that contain ground up iron/steel.
In future it may be a good idea to have a couple of buckets of water standing somewhere out of the way with a cover on it. I am not a fan of those chemicals to condition water, prefering to limit changes to 1/3 per week.