Steel tanks are definitely magnetic.
The Navy Explosive Ordinance Division (EOD) and Underwater Demolition Team (UDT, now SEAL team) have always used aluminum tanks to avoid any magnetic signature. Magnetic mines are extremely sensitive. They are design to sense a ship a distance away. The tanks of EOD or UDT diver are small (compared to a ship), but he may need to get right next to the mine. Even their regulators were not chromed to avoid any magnetic signature. The internal components were gold plated and the case was painted black.
Your compass is not as sensitive as an underwater mine and therefore, not affected as much. The distance from your back to your compass should be sufficient to isolate it. But, you can easily test it. Move your compass in a straight line towards your tanks. Notice at what distance you can see the compass card deviating a substantial amount. It should be very obvious.
This could be another good reason to use aluminum tanks for side mounted (slung) cylinders (deco bottles, ponies, etc.).
Also, you don’t normally care about small deviations since you are using your compass to follow a straight line, not a precise compass course based on a marine chart.
