Hi @gaynord,
A warm welcome to the Bunnings Workshop community and thank you for your question.
I imagine the tank will have drained or is close to draining below the level of this hole which will enable us to get a better picture of what we're dealing with.
I'd start by applying White Knight 0.125L Rust Guard Rust Converter to the rusted areas. Rust converters are chemical solutions that transform iron oxide (rust) into a more stable compound. Using this will create a tough barrier that seals the surface, protecting it from moisture and oxygen which are the primary drivers of rust formation.
Once treated, with a rust converter, use Dulux Metalshield Zinc Galv Spraypak Grey to help protect the area of treated rust from further issues.
If the screw that has popped out was connected to a frame behind it, you may be able to use a slightly larger screw back through the hole to seal the hole, but this would rely on the frame behind the hole being in sound condition.
If this is not an option, the only solution I see working long-term is to weld a new piece of metal in place to cover and seal the gap. You might like to use a service such as HiPages or True Local to find a welder in your area who can assist.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
