04-18-2009, 02:16 PM
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Rocks from the 10g can be put in the 75g (don't wash them, as this will remove the good bacteria on them). I would not use any of the existing water, there is no significant benefit. The 75g has to be cycled, and I am assuming from your experience with the 10g and 20g that you know about the cycling process, so I will only summarize and mention how many fish as you asked.
When the 75g is set up, water at the correct temperature, etc (best to let the tank just run like this for a couple of days to ensure the filters and heaters are working properly, no leaks, etc). [By the way, two heaters are recommended for a 75g, one at either end. I would suggest 150w or 200w heaters; I have two 200w heaters in my 70g. Having two heaters means less wear and tear on each one and thus more reliable operation.] Add a few fish, the cherry barbs from your list would be OK, and dose the tank with "Cycle" or a similar biological product. Cycle reduces the stress of new tank syndrome on the fish and gets the bacteria cycling going faster.
Monitor your ammonia readings each day, it will rise and then start to decline; at that point, test for nitrite as well as ammonia daily. The nitrite will also rise and then decline. When both ammonia and nitrite read "0" for several days consecutively, the tank is "cycled" for what it contains. You can then add a few more fish (another 6 or so, say the black skirts and then the white skirts), wait a few days for the nitrosomonas and nitrobacter bacteria to multiply to the level needed to handle the additional bioload, then add a few more fish, etc. Cycling takes 2-8 weeks; just don't add too many fish at one time, as this will be more than the existing bacteria can handle and cause a mini-cycle to occur, stressing the fish. Bacteria multiply quickly once they are established, they divide in half (called fission) as the food (ammonia for nitrosomonas and nitrite for nitrobacter) increases, but you have to remain within a reasonable level to avoid crashing the biological equiliabium in th tank.
One last point, I would get a few more neon tetras to add to your existing 4; they are shoaling fish and groups of 6 minimum, preferably more, are best, and your 75g will easily handle another 5 or 6 neons to the rest. Good luck.