Various electronics companies make air pressure sensors. They often have simple analogue voltage outputs proportional to the pressure applied. Attach one of these sensors to pipe (I'm thinking small diameter silicon tubing) and submerge the remaining open end to the bottom of the tank whilst keeping it vertical so that no air bubbles out. The fluid pressure will compress the air in an attempt to match the level of the fluid inside the pipe, to that outside. The pressure will be proportional to the depth of fluid.
The advantage of this is that all electronics can be kept outside of the tank, so it will be safer to use in explosion risk zones.
For example...
http://www.freescale.com/webapp/sps/site/overview.jsp?code=DRSNSPRSSRhttp://content.honeywell.com/sensing/prodinfo/pressure_all/Check the electronics suppliers such as (amongst many others)
http://www.farnell.comhttp://uk.rs-online.com/web/http://cpc.farnell.com/ (different to Farnell, but owned by them. Cheaper stuff, more household / white goods spares)
http://www.rapidonline.comhttp://uk.digikey.com-Tim
