11-05-2008, 03:02 AM
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That article willow listed is a very good start for learning about the aquarium cycle, but if you have any more questions about it, please feel free to ask.
In case the article doesn't put it strongly enough, the aquarium cycle and the water parameters (ammonia, nitrite and nitrate) that it revolves around are extremely important to the health of your fish. Low levels of ammonia and nitrite can cause irreversible damage and can kill your fish over time, while even moderate levels can be fatal in a very short amount of time. Nitrate isn't as harmful but definitely has negative effects on the health of your fish in the long run. The best way to monitor the progress of the cycle (and thus be able to know when to do water changes to save your fish from an early death) is to get a good liquid testing kit. The API Freshwater Master Test Kit is a good choice as it offers a good balance between accuracy and user-friendliness. It runs for about $30 in stores and about half that price at online aquarium supply stores like Drs. Foster and Smith and Aquariumguys, but is definitely worth the investment and is essential for any fishkeeper.
As for your three survivors: the danios are very hardy and should survive the cycle even if you have a few minor hiccups along the way. The swordtail is a hardy fish but not so much as the danios, so you'll definitely have to keep on top of water changes during the cycle in order to keep him happy and healthy.
Swordtails are not schooling fish but wouldn't mind more of their own kind. If you choose to have both sexes, know that they *will* breed and females you purchase will likely come from the store already pregnant. They can quickly overrun a tank, so have a plan on what to do with all the babies when they come. If you want to avoid this issue, only get male fish. If you get both sexes, aim for a ratio of one male to every two or three females. Failing to do so will result in the males relentlessly harassing the females to the point of stressing them, perhaps to death. Of course, you could just stick with the one fish; he won't pine away if he's the only swordtail in the tank.
The danios, on the other hand, are a schooling fish and should be kept in a group of at least six. They'll feel much more secure this way and will be far more interesting to watch.
A Jack Dempsey will eventually need at least a 55g tank so isn't suitable for a 30g in the long run, although a young one could be housed in a 30g tank for a while before upgrading. I currently have a 4" Dempsey in a 29g tank by himself that will be getting an upgrade in the future. Dempseys are not at all good community fish and will eat anything small enough to fit in their mouths. Even if the Dempsey is small, he will likely kill your other fish. Very beautiful and interesting fish, but they don't play well with others.
The other non-community fish I listed on the first page are also not community-safe and will kill or eat the fish you've got.