Although that was definitely a lot of weight, I think it was more likely the nature of the load that threw you off. Large amounts of liquid slosh around while being transported and will easily upset a bicycle's center of balance. That's why truck drivers who haul tankers need special licensing (at least they do where I live). You would definitely need to carry a load like that low on the bike, or better yet get a trailer like others here have suggested, especially if you're truly going car-free.