You can use any fish to start a cycle, a lot of places use Eels because they're very hardy and can take a cycle, but there is no reason, and I repeat,
NO REASON to use a live fish to cycle your tank. All you need to cycle your tank is a source of ammonia, which can come from many place so there is
NO REASON to use a live animal to cycle your tank.
Sources to start cycle your tank:
Live Rock, the die off with give you the ammonia needed to cycle the tank.
Throwing raw shrimp,clams or a dead fish into the tank, either will give you a source of ammonia to start the cycle.
Fish food, flake or pellets, will give you a source of ammonia to cycle your tank.
It all comes down to adding a source of ammonia, and the more the better. Now a larger source of ammonia will take longer to cycle the tank, but your bacteria level will be much higher and it will be able to handle a much larger load once you start to add livestock to the tank.
In the mid 80's people use Ammonia Chloride to boost the bacteria levels during cycling, which you could get at a chemical warehouse. This was added during the cycle, just as the ammonia level was starting to peak. It would actually turn the ammonia test kit sample black and it take a couple of weeks for the color to come down to a level that a test kit could actually read, but in time the tank would cycle and you would have a super high level of bacteria, which could handle a tremendous load and would let you almost fully stock the tank from day one.
