I have had this happen a few times recently as some of you have already advised me and I appreciate any help and advice. My toxin levels have finally stabilised and my overly ambitious stock reduced to a more sustainable level, (7 male guppies in 10 gallons), I’ve added many more plants - anacharis and water sprite, I feed every other day. One of my poor fish has once again deteriorated. I have been trying to treat his fin rot for weeks with water changes and Melafix. It has visibly worked well for the two others who had fin rot including one who has grown back a large bite out of his tail without trouble. this poor guy has just been getting worse, along with his tail rot he has been losing weight and energy and can barely support the weight of his own tail in the water. now he is taking frequent rests on the substrate. I do water changes twice a week but he always seems much worse after the stress of the water change so if possible I don’t want to be doing them every day again. I don’t understand how the treatment could work on the others but not him. The other possibility I have pondered is that I bought my stock from two different sources, all the ones that have weakened have been from the same breeder and all had big tails and cobra patterning. All my very healthy looking fish are round tails, brightly coloured but not so fancy. Should I find a stronger medication or change my maintainance schedule? Are the positive effects of the new plants just taking time to kick in? Has anyone actually had a guppy make a full recovery after they have been lying poorly on the bottom of the tank? How likely is it to be genetics and not my care compounding the problem?