Dosing pumps

[zohsix]

This is to do with number of rollers in the peristaltic pump. A cheap 3 roller peristaltic pump, can allow liquid to reverse flow back through the pump, as there is a pump rotor position that when stationary still allows fluid to flow through, especially if you locate the pump below the fish tank. A flow valve can help, but with fertiliser will have a habit of regularly blocking.

A 4 roller pump does not suffer from this as when stationary there will always be a roller pinching the tube preventing reverse flow. 4 roller pumps give a much more precise dose.

This is not my experience. I had a four roller pump that was installed below the tank and it back-siphoned. I now have a two roller pump that does not back-siphon. I understand your post but I also think there might be other explanations, such has how tightly the roller pinches the tube. My suggestion would be to not buy a pump unless it's stated that it will not back-siphon. I didn't do that the first time and threw away $160 and a boat load of time fussing with it.