tank is a 55 gal with 30 gal sump/fuge, 4 fish in it are 2 clowns,1 blue tang,and a royal grama. temp is always 80 i use a skimmer,3 powerheads,filter sock,and have a 4-54watt t5 setup on timers.. blues come on at 10:30 am then daylights come on at 11:00am then daylights go off at 9:30pm and blues go out at 10:00pm.. i keep getting this slime build up on the sand,but it is only on the left side of the tank,and never in the fuge. any idea why its doing this??