Rocks for 55 gal.

That is a pretty vague question.

With adding rocks to an aquarium there are usually two main concerns:

1) What type of rock are you adding?

Some rocks can affect the PH of your tank, which in some cases is not desirable. In some cases people add limestone rocks to increase the PH to recreate natural conditions of some fish. Like Lake Malawi Cichlids. Though this is most often not necessary to non-wild caught fish.

2) Are you stacking the rocks?

What effect or design are you trying to achieve? If you are stacking the rocks you need to make sure that they will not tumble off the stack causing your tank to crack/shatter.
 
In that case, the only thing I could say would be it's your personal preference.

Do a search on Malawi aquariums. The natural habitat of Lake Malawi Cichlids is mostly Rock/sand with very little plants, if at all. They often have pics of their tanks showing a lot of rockwork.

That may help with ideas.
 
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