[Lower TDS]

I know that less feeding less and weekly water changes will lower the TDS but my question is, should I continue to mineralize the R/O water or just leave it plain? Parameters are good and I change 10g’s a week out of my 55g planted tank I also dose ferts twice a week of which the 2nd dose is after a water change. . My TDS is 250 and I’m trying to get it down to 150. Any advice would be greatly appreciated. 

[Help!!! Fin issue. Not sure what this is.]

I can’t figure out what this red thing is on the tail fin of my Male Black Skirt tetra. I’ve tried google with no luck. He’s the only one in the tank with this issue and I’m not sure if it’s contagious or how to go about treating him. Any idea what it is??