This issue is not uncommon with aerator pump based systems. Aerator pumps do not block the flow of water when they are turned off. This is an inherent function of their design. As a result, if the thru-hull fitting on the side of the boat is lower than the water level in the bag, passive draining will occur.
There are a few possible solutions:
- Relocate the thru-hull higher on the side of the boat (not usually something that can be done).
- Route the hose from the drain pump to the thru-hull fitting as high up on the side of the boat as you possibly can to get above the water level in the bag. This will almost always solve the problem, but may also require an Anti-siphon Vented Loop (https://www.wakemakers.com/vented-loop-anti-siphon.html) to break the siphon that can be established.
- Install a manual ball-valve in the drain line that is opened when draining, and closed the rest of the time.
- Manually plug the thru-hull to prevent the draining from occurring.
- Run the fill pump as needed to keep the bag full.
Contact a ballast expert for more information if you are dealing with this issue on your boat.