Well it sounds to me like your water source has those nitrates in it and that you are replacing them during water changes. If your lfs
is telling you that spending money in their store will lower your nitrates, which are at a fine level for a reef tank for the record, then Iíd pick a different place to shop and get advice.
A 55 is a fine tank for a reef. I had one running for several years before upgrading to my current system.
The key for reefs is stability. Youíll need to keep up with your weekly 10% water changes to replace the elements that the corals are consuming. You wonít need to dose anything as at such a size itíll cause more issues than solve.
Most corals in the hobby are photosynthetic. Proper lighting is all they need to survive.