You don't want tank water entering your dosing tubes. Build up, bacteria and "pulling" more than what you actually dose into the water can cause issues. Also, microfauna can find it's way up the tubes as well.We have the outlets from the tubes from the dosing pumps at the intake to the return pumps. At the moment we run a temporary aquarium with lots of holding tanks connected to the same sumps and fed by the same return pumps. This way all tanks gets some EasyBooster/SPS.
Worked well for about two years now. But of course we have to check the tubes now and then. Never had problems at the outlet, but a couple of times at the dosing pump and at the connection to the bag.
Whould you like to explain the reasons for that(tubes above the water). Thanks!