Local company to me uses fine sand blasting via filling point. Problem is - if the rust is more established - you can cause leak. Good luck
Bit of a science experiment. Old theory is fill it with small washed gravel(fish tank stuff) and swish it round for an eternity in your hands.(some people secure it in a cement mixer to do the work) The gravel removes the rust and then you have the devils own job trying to get shitty gravel outta your nice clean tank. Here's the new idea. 1.)Battery charger connected to 12 volt battery. 2.)Jump leads from battery connected. Negative to tank, positive to a diode(in this case, a 22mm wood bit)and placed vertically in the tank filler. 3.) Tank filled with warm water and soda crystals solution. 4.) Plug in and electrolysis begins(remember copper sulphate solution and nails at school.) It works.
I saw a vid describing this method with the electrode. I actually used the gravel, nuts and bolts with distilled water method and it seem to do the job. I haven't detected any rust in the filters, but time will tell and in the mean time will collect the needed items and give the electrical method a whirl. Thank you for for your suggestions....