I have two 15 gallon tanks and one 36 gallon tank. I had a issues with fin rot in my 36 so I put the fish in my empty 15 to medicate with melafix . Once they were well I moved them back to the 36 4 angels , a platy, and 2 cories. I have done a water change on the 15 and put the carbon back in but it smells horrible!! It start on day 5 of treatment so I thought it was because there was no filter. I was doing water changes every other day while they were in there. Please help it’s in my bedroom and I can’t stand the smell.