I rescued 3 guppies today (from family not the store). They were in cycled conditions in a 30g with rosy barbs, peppered corys, and platys at 72F. However the source water had 30ppm nitrates in it. The tank was regularly at 80ppm at the time of (recently) weekly changes. I think this constantly high nitrate reading is the cause for most of their condition. My tank is a 20H, 75F, cycled with 5-10ppm nitrates between weekly 50% water changes, and these three are joining 5 male guppies, 1 platy, and 6 Kuhlis. One is a female, so pale/clear that I can't tell her original color. She has a slit in her tail fin.Next guppy is a pink mosaic male, super faded with several slits in the tail fin and what appears to be bites missing or shredding.Final male guppy, I think is a sunrise guppy. He has only a sliver of tail fin left with white and gray fuzziness to the ends. I fear it is spreading up his body.My guppies look completely healthy except:1 seems to have a darkening/graying/blackening of his tail fin. I've been watching it closely and it seems to get darker then lighter depending on the day. I wonder if this is actually normal. However, after inspecting pictures, it seems to be spreading.And my other male guppy seems to have a broken ray in his tail fin. Is this possible? The bottom edge of it folds and flops around instead of maintaining a strong triangular shape.So, if I need to treat, I was thinking of treating the whole 20 gallon tank. I have a 10gallon QT that's newly cycled (and was saving for a shrimp colony that's on order and en route). The three rescues: My darkening male, taken about 2-3 weeks ago. This is where he is lighter. When he's darker the fin is almost black at the bottom and a bit on the tips too. The next one shows that it's spreading to the top, taken today. My broken rays. Sorry for poor quality. Guppies are tough to photograph: Meds I have on hand: Furan-2 General Cure Paraguard Tetracycline Kanaplex HELP! @CindiL ?