actuallly the cheapest is purchasing a straw which u could blow bubbles into the water with.. hehe just kidding.. if u're using a power filter (which is presume as you do not have an air pump) you could lead the water on top of the water surface so when the water hits the surface u'd see bubbles
You need an air pump, some vinyl tubing and and bubble want or bubble stone. You should be able to get the whole thing for under $20.00. You can make the bubbles in one area using a stone or have them running along the entire back of the tank using a wand. Check out this link to see one pump. You can just look at your LFS for an inexpensive pump or order online. http/www.drsfostersmith.com/product/sub....d=6&pcatid=3669
honestly u can do without them as u have a filter.. the pump does not really pump oxygen into the water but introdces it to the water when the bubbles burst.. since the filter will contribute to surface agitation u could save your money
Ah Joshua I am sorry - I didn't see your advice (I'm at work and sometimes log in if I have a few minutes spare!!!) Your advice was sound - I didn't get to LFS last night and will give it some thought before I go spending money.