I have 4 stony frags in my tank. They all seem to be doing well except for 1. I don't know what its called but the bottom of it is turning white and peeling a little bit. I saw a vid on youtube that said if this is happening your Alkalinity is off. (I'm not sure if it means its too low or too high) What test kit should I but and what do I use to get my Alk. back to normal?

you mean that you have sps and don't already test!?? bad move! do you dose alkanlinty and calcium?
Get a test for KH and for calcium.
KH should be between 8-11
Calcium 400-450
That should be a start to your problems
as far as additives for alk and calc ESV B-ionic works the best for me

it may be that something is just eating it. You really do need to test tho.

I use aqua tech dissolvable blocks and that's it. I will get a KH and calc test kit tho.

ive never heard of those blocks before. api alk is pretty good imo but dont bother with thier CA, im thinkin of just going all out and getting the elos

Blocks sounds like an inaccurate method of dosing, especially if you haven't tested the tank. It could be that your corals are experiencing tissue necrosis. Read this:
http://reefkeeping.com/issues/2007-04/rhf/index.php

Also, you need to get IDs of your frags. There are many stony coral families, and not all of them are very hardy with uncertain water quality.

I don't understand though. My three other SPS are doing great and growing. The one thats doing bad look like it is starting to erode from the base. My orange motipora has gotten twice as big from when I got it 2 months ago.

You can not use the other 3 to judge the one.

Monti's as far as my experience are hardy and grow well even in not so perfect conditions while other SPS are more demanding.

Just like stated previously... you need to identify the coral to get an idea of it's requirements.

because some corals are more sensitive than others to water quality problems. or, there may be no water quality problem and there's something else that's wrong. we can't tell you anything because you aren't giving us any numbers.

you need to test for ammonia, nitrite, nitrate, phosphate, pH, calcium, alkalinity and magnesium... Elos, Salifert, Seachem, API -- any of those are acceptable. Based on my own experience, I'd avoid Red Sea and Tetra test kits.

Here's a good coral guide.
http://www.asira.org/caresheets

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