I chose RV antifreeze, which is nontoxic, to load my rear tires (24x12x12 ag lug tires). Each tire took about 8 gallons if I recall correctly. Antifreeze is heavier than water at around 9.4lbs/gallon, so each tire has about 75lbs in it. Knock on wood, it's never leaked. The RV antifreeze won't corrode the wheels and if it ever does spill, it won't poison the ground. A nice side-effect of the loaded tires is they never go flat. Well, there's little to no air pressure left in them, but there's so little headspace that they don't deform due to lack of air pressure.

Regarding weight boxes, I built a rear weight box with a frame of 1/4" angle iron and sides of 3/4" plywood. The box has 3/8" thick "prongs" that go into the rear stab holes. The box weighs about 100lbs by itself, and has a hinged lid. I got buckets of used lead wheel weights from a tire shop for free, and loaded them all into the gallon jugs the RV antifreeze came in. I built the weightbox to fit 12 of the gallon jugs, and they weigh around 32lbs apiece. All told, the box and lead-filled jugs add about 485 pounds if I load it up fully. I only use this much weight if I'm using the loader to maximum capacity and need the counterweight to lighten the front end so it's possible to steer. Otherwise, I use about half the weight, distributed towards the sides for a greater moment of intertia. It's not too bad to install and remove the box, as each jug is easily moved by itself and it's not too bad to slide the box out of the stab holes and onto the ground, or vice-versa.

Regards,

Ben