They're relatively expensive, but for some obsidian and glass you can have pretty big tanks thanks to OpenBlocks.
EDIT: Four obsidian and 5 glass panes, to be precise, gets you two tanks.
Which seems fairly reasonable, really. Each tank holds 16000mB. Which I think is 16 buckets, unless my math is completely off. Which it shouldn't be. Metric system, and all.) But since it's modular, they're pretty easy to expand. It's may not be an improvement, but they look pretty neat.