Why is sand filter blowing algae back in pool while vacuuming?

Im not an expert by any means but what I know is that with a sand filter system if your pump is not the corect HP for the filter it will push dibris and sand right back into the pool

answer::

That may be true in remote cases where the system was not installed correctly or had well balanced/matched equipment. The correct answer would most likely be that you have a broken lateral(s) which allow sand to pass back to the pool. It is best to find the correct type lateral for your specific filter. Some manufacturers have made modifications to same model filters as production and technology progresses. Replacing the lateral required turning the equipment off and making sure it will not come back on while it is being worked on. Removing the filter top or valve assy. Removing all sand, finding the damaged lateral(s), replacing lateral, replacing the sand with new sand in the correct manner, replacing the valve or filter top and starting the equipment and backwashing the filter to reduce the dust blow back

K