I use neon tetra's to help cycle my tanks. they are cheap and quite tough. but 4-5 days is not enough time to cycle a tank. You need some kind of BIO waste products to get the tank cycling some people add ammonia directly to a tank to give the nitrifying bacteria something to munch on. I have never done this, i do the following:
if started totally from scratch, aka new everything. I run the new filter in one of my established tanks for a week or two so the bacteria can build up in the filter media. then transfer the filter and everything in it to the new tank. then add some cheap fish and and run that config for at least a month, all the time watching the ammonia levels. Then after a month i start adding more fish.
If I want/need to speed the process up i take substrate from an established tank and use that for the new tanks substrate. I take the substrate and all of the gunk that was in it that's the good stuff!.
if this is your first tank put the tetras in and keep an eye on the ammonia levels. If they get too high do a partial water change[10%-25%] otherwise just let it spike. do yourself a favor and get a test kit that has Nitrate, Nitrite, Ammonia, and PH tests in it. Test Strips are more expensive but are much easier to use.
Here is a white paper on the nitrogen cycle, it salt water based but the basis is the same.
http://www.reefscapes.net/articles/articles/2002/nitrogencycle.html
is this you first tank?