ok, well when was the last time you changed yhou canister media? how often do yu change your canister media?... canisters have a tendancy to cause high nitrates.. but i agree with jimmy..100 seems a lil too high.. but if it is correct, ouch.. spend a few bucks, buy yourseld a new test kit(just to be sure) buy a HOB skimmer.. u can still use your canister but you should use it like a reactor.. remove all the bio media, use carbon and gfo(granular ferric oxide,also known as phosban), and you can use a seachem Purigen filtration pad in the canister for help with the absorbtion of amonia, nitrites and nitrates by removing organic waste(and its reuseable, 1 pad is good for upto 100g)..
i dont know if you do or not but do not disturb the sand bed when doing water changes it should be left alone..
also if you plan on getting corals, you need a good lighting system, not sure if you said what you have, and also RO/DI is not a sugjestion its a nesesity for coral health and growth(never use any copper treatments)
so with that said.. try that before you go crazy building/buying a sump.
so 1. get a skimmer
2. get the bio media out of the canister
3. use good carbon, gfo and purigen in your canister
4. use hydor korralias for water flow should be atleast 10times your total gallonage
5. NEVER LISTEN TO PETLAND!!!!
good luck