either use up the solution in the res or dump it. Clean the res with a bleach solution and ad fresh solution. If the res has set for any amount of time, even dry and empty it needs to be disinfected, even when new. Nothing else makes sense when therew are those of us without this problem. The small amount of sediment is not an issue. My last grow I never cleaned the system at all during the grow, no line flushing or changing, nothing. And I've not had a problem with scum/algae, but I clean and disinfect the res, lines and trays the day I start the res.