hi, i made some really great ones for an aquaculture project years ago:
here is my technique:
inner mold of corrugated iron overlapping- spring this cylinder out with the help of a friend and use a double row of tek screws to hold it.
paint with veg oil or soap as a release agent.
now you need to put some thin strips of wood standing vertically between the inner mold and your ferro material, i used chicken wire for the ferro sheet. these thin laths will be withdrawn as you pour, and used to consolidate the pour by wiggling them.
wrap with ferro.
another layer of laths.
wrap the whole thing with a greased sheet of flat thin steel sheet, and screw fasten.
now pour around and around with a rich mix- use plenty of fat sand, lime, and cement, NOT too much water! i always use a bit of detergent in the mix to reduce water.
withdraw the laths carefully as you pour in order to keep your steel centralised.

you will not believe how thin you can make the walls and how cheap and quick to build. a few ppl working on this can accomplish an almost magical amount of work.



i like to pour a slab with a channel for the mold to stand in, then pour the actual tank.
roof with steel and provide input filters, coarse sand and good charcoal. a dirty tank is a nasty health hazrad and mostly they are dirty when not properly isolated from pathogens.
clean in equals clean out.
cement is a beautiful storage media for water.

give me a shout when you're ready and maybe i can give you a hand.