The reason that de goes backward is because the filter is usually higher than the pool. If you allow air in the return plumbing, then the filter drains backward through the suction side. You can put the valve on closed, waste or recirculate to prevent de from draining out of the filter. Or, you can close the skimmer and main drain valves.
Another tip: Watch the returns for a few minutes when adding de to see if any is bypassing the filter.
Another tip: Watch the returns for a few minutes when adding de to see if any is bypassing the filter.