i cant get my phosguard to tumble either. but on their website they mention that due to the spherical shape of phosguard it allows current to flow between granules easier. that my be why there is less tumbling. i use phosguard over gfo. i was have some big issues with GFO which happened suddenly after using it for well over a year.

phosguard is aluminum based, but you need a pH of 5.6 to get it to leach into the water. if your pH is that low you have bigger problems them aluminum.