Large areas of simple fluid tanks are very slow to equalize fluid levels across themselves - this makes it very difficult to store very large amounts of fluid that is evenly distributed, and can be quickly pumped out (and in)
This system uses the circuit network and pumps to actively manage fluid levels across the entire tank reservoir - fluid is maintained at even levels horizontally, and flows downwards only overall (where top is input and bottom is output). The design is fully tileable.
Note: I have no idea how many combinators will start to cause lag, and the standard non hysteric design will keep the horizontal pipes in more or less constant toggling of on/off, so i included a design that uses hysteresis in the horizontal direction to solve this - the disadvantage being that the fluid level in the horizontal direction can vary by as much as 200 per tank.
IMPORTANT: This design is configured for steam - if you want to store another fluid you need to change the fluid measured by all the combinators