Keeping these in one place:

nitrofenix: i'd base it on banks but maybe with half the amount of slots per tier
trunksbomb: 8 buckets/16/64/256/1024/1000000

Tiers/capacities from the old mod (for reference/discussion only):

1: 3 tanks of 4000 mB each
2: 6 tanks of 16000 mB each
3: 9 tanks of 64000 mB each
4: 12 tanks of 256000 mB each
5: 15 tanks of 1024000 mB each
6: 18 tanks of 4096000 mB each
7: 27 tanks of 247483647 mB each

Huh, would you look at that - I was pretty spot on for capacity :)

Additional suggestions:

• Lockable slots
• Automatic pickup of fluids when standing in them? Probably not a good idea
• use the tank itself as the bucket (no dedicated bucket item like entangled buckets)
• shift + click air to change modes, shift + scroll while in "build" mode to change which liquid you're selecting
• definitely a "void" mode to void excess fluids
• maybe a sponge mode, or a mode that functions like a hose pulley from Create so you can target one block of fluid and have it suck up other blocks in the same body of water
• shift+click bucket of fluid in inventory while Tank GUI is open automatically inserts the fluid from that bucket, like Modern Industrialization machines

Done so far:

• tank 1-7 and dock
• can interact with buckets/similar in gui including shift click
• can interact with pipes in world using dock
• can lock slots

Heck yes, might give it a test run if I get some time over the next day or so

It's ready for testing! Recipes and number rendering is left to do. jar can be grabbed here
I am also thinking about reducing the number of slots per tier but upping the capacity. I think most slots will be unused unlike banks. And in vanilla there are only 2 fluids.