03-28-2009, 12:59 PM
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From earlier posts, you have a 20g that has been running for a month, it contains 3 platys, and water tests are good. Yes, the tank is cycled for what it holds. I wouldn't recommend adding 6 neons and 3 corys at once. Add the 6 neons, wait a few days, then add the corys.
This is a small ecosystem. The nitrosomonas and nitrobacter bacteria have established themselves at a level to handle the available food they need. Suddenly increasing the bioload will mean more ammonia than the nitrosomonas can handle, and subsequently the nitrobacter--and that means you will have first an ammonia and then a nitrite spike and at the least stressed fish (which could lead to disease) and at the worst fish loss. It has to be gradual, especially in a small tank. The bacteria reproduce by splitting in half (called fission) and can do this fairly quickly when the food source (ammonia, or nitrite as the case may be) is present, but not immediately. Of course, the larger the tank and the greater the established bioload, the easier it is to add new fish.
And I would add the neons first, then the corys, only because corys are very sensitive to water conditions and will have less of a shock with a more mature system. Yes, this will be a very nice display. I didn't notice if your pH was mentioned; neons and corys prefer slightly acidic (below 7) but can be acclimated to around neutral, whereas platys prefer alkaline (above 7), but should be OK at neutral.