That's a common problem with our aluminum water tanks. To minimize the problem, get a carbon filter from Lowes or Home Depot, and rig it up so you can connect a hose to it when you fill your tank. No chlorine!!! is the cardinal rule. The "crud" you're seeing is almost certainly aluminum chloride.
I went a step further and installed a whole house filter between the tank and the water pump. I use the coarsest filter available, and I've had no problems with blockages since doing that.