ok i have a 5 inch albino tiger and a 6 inch greeen severum and 4 2 inch silver dollars in my 75 gallon all of a sudden my oscars fins look horrible but everybody else is fine my ammonia is 0 my nitrites are 0 and my nitrates are 20 , i dont get it i did a water change and put some melafix in it almost looks like lil bite marks could this be the silver dollars what yall think what do i do