My fish keeps rubbing against the rocks in my tank ...

It maybe a case of "fish Ich".

yeah. sounds like ick/ich.

get some medicine for it and read the instructions properly. although, i would go against some of the instructions and treat the fish for at least a week with the medicine. also, it helps to raise the temp to about 82 degrees, if your fish can take it. and, if you separate the fish from the others in another tank/netting then you can help avoid spreading the parasites. 

actually im afraid your both wrong . what the fish is doing is just scratching itself on the rocks. if it has ich/ick them you would see what would look like white fluff of the infected areas. the way to cure this is with aquarium salt. you add 1 teaspoon per gallon of water and it should be cred in 2 weeks if you do this correctly. also it helps to turn up the temp a little bit.

inspect your fish! any salt like spots? if it looks perfect then have the water chemistery checked. ph, ammonia, nitrites all burn at their scales and they think they can rub the pain away. I managed tripical fish stores for 12 years the fish farms for 6 more. if there salt water fish it's worse, the ocean doesn't change so those fish die faster, their just not used to it. hope this helps

i think thier is a madicine that we can buy in a pet shop