Probably still need a bit more info., mate...
Are you refering to some rust in the guards (just above the wheels)... If so, this is a pretty common place for EA's to rust.
The only REAL way to fix rust is to take it to a panel beater and get them to cut out the rust sections and weld some fresh steel in it's place... Anything else you do will only be a temporary measure. Also, fish oil is more of a preventative measure than a treatment, perhaps if you buy some rust converter and metal mend from autobarn/repco/bursons/etc. you might have more luck fixing it yourself.
Once it's fixed, it's a good idea to keep the undersides of the arches clean and free from dirt/mud. I do it everytime i wash the car - just hold the hose hard up against the inside of the guard, and let the water pressure do the work for you. Having water get into there is not the problem as water can just run away - as long as the water does not get trapped there by piles of mud!
Hope this helps.