That is a common enough issue for water systems, and an easy enough issue to fix.

What you need is a loop of pipe raised above the maximum water level and a anti-siphon valve installed at the high point. This should be located on the discharge side of the pump.

If you want to save a little money it is simple enough to avoid having to buy an anti-siphon valve and simply drill a tiny hole in the top of the loop. The hole only has to be big enough to allow air in and you can get creative with the tiny fountain coming off the top of the loop when the pump runs.

An interesting variation is to install hose barb at the top instead of just having a hole. From there you run a bit of vinyl hose to something that needs watering. Or even just back to the tank. If you do use some tubing you will have to make sure it drains by gravity without any bellies or obstructions so it will allow the air to flow back up and break the vacuum on the loop and stop the siphoning.