I only use it a couple times in the spring, so filling with plain water it isn't a problem.Mine's poly, so no rust issues.
You don't need to worry about that with RimGuard I don't believe. None toxic food product. Antifreeze I agree with.What if you spring a leak with antifreeze, rimguard etc. ? Kills a bit of grass, you dig it out , get some clean dirt reseed it....