cycling.....................

The Cycling thread in the Freshwater Newbie section that is a Sticky near the top of the forum explains the whole process in detail. For the Cliff Notes version: Test the water everyday. If you see ammonia or nitrites, do a 50% water change. After a while you'll start seeing nitrates on the rise, when this happens you know the cycle is coming along. Keep up daily testing. Eventually you will start getting readings of ammonia and nitrites of zero. If they stay there, then your tank is cycled for its current bioload. What this means is that your tank is cycled for all of these smaller, cleaner fish. If you put 6 pirahnas in at one time, you will force your tank into another cycle. Also, the crayfish is likely to eat a fish if it can grab one, so it may be a bad choice for your tank. Personally, I'd have done a fishless cycle. ANyway, there it is condensed, you'll still want to venture into that other forum and read about the whole process. Best of luck, and let us know how it comes along or if you have any other questions!

If you let the mollies reproduce enough to fill your tank, they should eventually get your bioload high enough... and then the pirhana's will eat them... of course, ya gotta question whether eating THAT many fish in one day like they will will be healthy lol.

Chances are ina tank taht small you may find it hard to get any reaings with that group of fish. It may cycle without any large spikes. I assume you want to let the mollies and tetras get eaten. If you are raising molllies as feeder fish (and it is best to raise your own) then I suggest getting a smaller tank (20 gallon or so) and rasing the mollies in their so the other fish don't eat all the babies at once.