OK, so no corals...good.
Adding the LR to a tank with fish can be dicey, as the rock needs to 'cure' or get a buildup of bacteria to work as your filter...there is usually die off of sorts from the rock as well, causing the ammonia spike.
Alot of folks will cure their rock in a separate rubbermaid tub or other inexpensive container with powerhead and water changes for about 4 to 6 weeks ..(for future reference)
Keep the temps on the lower end if possible, around 76 if you can. Ammonia becomes more toxic with higher temps.
Is this the filter you have..
http://www.tetra-fish.com/catalog/product.aspx?id=283
If so, you will definitely need to upgrade your filtration. That is barely the right size for your tank (up to 30 gallons). Plus, I would not keep that as the sole source of filtration. You should invest in an under the tank 'wet/dry' if you will keep fish only. Good Protien skimmer if you want to keep corals eventually.
You will also need alot more flow to keep the tang happy (which may be difficult) see here for info on tangs..
What about tangs?
They get very large, fairly quickly , are fast swimmers, and need room. Plus, they poop alot, so are a heavy burden on the filtration setup. For more flow, you might need to add powerhead inside the tank.
I hope I am not overwhelming or discouraging you! The Liverock is a step in the right direction! Keep reading and posting questions!
There is a decent fairly new book, The New Marine Aquarium by Mike Palletta (I can't remember if the word 'new' is in the title, it may not be) that is very good and relatively inexpensive, to get some solid basics from.