Pics will help, Cecejay! If you need help with posting them, please ask!
Although the temp at 24c is high, I do not believe any of your issues are direct result of solely this. High temps though can cause discomfort such as bottom sitting or clamping but my thoughts are the issue is your water source.
Ok, let's go back to some things you mentioned. Did their symptoms start when you added the UV? Remove it completely until we can get to the bottom of this just on a rare chance something is leeching from it. Or did it coincide with a water change? I suspect something in your water is the culprit as you said you did another wc and they have more issues today. When did the fluoride addition start? Did their symptoms begin with this? Fluoride can be toxic to aquatic creatures and the sources used for fluoride 'enhancement' in public water supplies are quite questionable. Have you contacted your water authority to see what levels are being added, what source is being used for fluoride or if anything else has changed with how they process your water supply? Is it possible to have your tap tested independently by a private company?
Your fish are definitely NOT under going 'normal' color change. They are exhibiting classic signs of chemicals burns. You have had them for quite awhile thus negating the possibility that a new fish introduced a pathogen. So, I am going to go out on a limb here in my suggestion if budget is not a major constraint. Invest in an RO unit for your sink. The water will need to be properly reconstituted with buffers and minerals such as Seachem Goldbuffer and Seachem Replenish. Switch them over to RO. It will eliminate the fluoride and any other potential issues your tap water seems to be harboring. They may need some additional intervention to help clear up the bacterial issues that are now present but I am not sure of the antibiotic availability in your country. Anyway, its a suggestion but it may not be a reasonable one for you to consider. Rainwater may be another option to consider (reconstituted) that would be less expensive. If you have brand new carbon or purigen, I would run this now and replace it every two days or so until you can figure out what exactly is going on with your tap water. If you have antibiotics available, I can make some suggestions, too. I wish I could offer something more concrete here but these are the best guesses I have right now with very limited information on your water source itself or what may be amiss.
