Im a DIY expert.
The yellow GPZ tank is an upsidedown horseshoe, which carries I think about 13 or 19 litres. I forget which.
It had been sitting in a shed sometime before the Big Bang and was badly rusted. After doing a lot of research I will give you the options I tried and their results.
Hydrochloric acid (sometimes called myriatic acid on American sites)
Weaken the acid in water and slosh around. Leave for 20 minutes to a few hours, depending on the strength of the solution.
Rinse out quickly and completely. Either spray with WD40 or use a baking soda and water mixture to neutralise any remaining acid.
Hit with an air compressor to dry.
As soon as remotely possible, get fuel back in there and get going.
Result: HCL is insane on metal at full strength. But if you dont clean it off - and we're talking REALLY clean it off, it keeps on working
Holes occur - not cool.
A less metal hungry solution is phosphoric acid and that takes longer, but works in a more gentle fashion.
FORGET KERO AND THE NUTS AND BOLTS OR CHAIN OR SAND OR ANY OF THOSE THINGS. The only reason Im saying it is because you dont want to be shaking shaking shaking for 40 minutes and the trying to get all that crap out of there. Its labour intensive and hit and miss in my experience.
My latest idea has been to use anodizing. If you google up anodising rust from a fuel tank, you'll see many articles. Essentially, buy a threaded rod from bunnings, fill up your tank with water and baking soda, connect up a small charger and put the rod in the water. Once you hit the power, the rust is attracted off the tank onto your rod. The rod becomes rusty, the tank becomes clean. No acid and no further silliness.
The only problem with it on the Z is that due to the shape of the tank, its hard to get a rod down there (even a bent one) without touching the sides of the tank, which ruins the reaction. Another downfall is there is a black residue on some parts of the tank afterward. These can be easily removied with a hose or whatever.
And what option did I take? I bought a secondhand GPZ and stole the fuel tank (and other bits) off that. Rust in your fuel tank is not fun. At all.
It may be different for the CBR250 (I cant remember the sinde of mine), but on weird shaped tanks, either leave it to the pros or get yourself another.