I would ditch the check valves all together.
for back siphon problem. If you can't get a new sump. Put an overflow drain in main filter sump that drains to a reservior that holds excess volume, install a reverse ato system. Power returns and a small MAxijet pump pumps water back to main filter sump and have a float switch power off pump when reservior empty.
Not a bad plan and easy to impiment^^^You joined in 2006 and this is your first post ever?
Thanks for help. I feel obligated to honor your first post wishes.
Tank was almost clear enough to see all the way to the other side today. Then I stirred it up and its back to being cloudy. Emptied the protien skimmers cup four times today, pulling out thick milk shakes of dusty water. I should of rinsed the sand even though the bags told me it wasnt necessary.
i always rinse my sand like 3 times just to cut down on dust