Alex, I used windshield washer fluid to fill mine. Bought a nifty little filler attachment at NAPA. You raise a rear wheel (one at a time, please), pull the valve core, attach this little adapter and then pump or pour in the fluid. My BX2200 holds 5 gals in each rear tire. Made a world of difference in traction when using the FEL. Not so good if you're mowing a real lawn (I'm not). I've not had a leak in the 3 years I've had it in. And if it did leak, no problem, it's non-toxic. No worries about it rusting the wheels either. And way cheaper than almost every other alternative.