Dwarf Puffer Tail Is Losing Color Looks Weird

[neon_platy]

ok my dwarf puffers, lost one and it looked the way the other one who has survived the longest is starting to get...i never noticed any fighting, but the tail is white and red looking injured or some problem...i don't have a test kit but i just cleaned the tank and i put in the bio spira stuff the other day and there's a ton of plants, caves, and a pot for them to hide in so i don't think they did this to themselves through fighting...any ideas??? I know they're hard to take care of, but i figured with the bio spira anti ammonia/nitri(a)te stuff i've put in it wouldn't be from that...if i can get this one's tail to heal or whatever the prob is so he lives, then i plan on just keeping the one puffer in the tank...any ideas??

 

[WILDER]

Need to no size of tank in gallons or litres, plus how many fish and which type, and water stats in ammonia,nitrite,nitrate,and ph.
Red tails can mean septicemia, any red edging to the tail, or red sreak running through it.