Algae in fuel

[RJBKSU]

We have a farm truck we bought in 2017. It is used and has 500,000 miles on it. International with isx cm 870. We fought an algae problem all through harvest adding a treatment called killem to get rid of the crap and changed filters when needed. We add the product at every fill up and try to keep the tanks full as possible. Never seemed to clear up so before harvest this fall we drained both fuel tanks and filled with clean fuel continued using the product at every fill up and we are religious about adding Howes at every fill up also. It helped a little but still have fuel with a black tint to it after we started running it again. Could it be the stuff is on the inside of the tanks and they need to be removed and steam cleaned out or just keep running the product and it will clear up eventually. This is what some people we have talked to have said but i am not convinced. Or is there a better product to use. Thank you for any replies.