03-11-2011, 08:57 AM
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I'm glad you read the article on cycling a tank - very important step. As you've seen, you can lose lots of fish in a non-cycled tank.
As far as how many fish you can put in a 20g tank - in general, you can use one inch of adult fish to one gallon of water as a rule-of-thumb. However, some fish need more water than that, because of territorial needs, being more active, needing to live in a school of the same species, or whatever. I STRONGLY recommend that you know EXACTLY what kind of fish you have, what their species needs in terms of food, lighting, space, male to female ratio, etc. Check out fish info on this site and others on the internet about what you have, and if you discover that they won't be compatible in the long run, then either take them back to the store and exchange them for something compatible, or get another tank (or more) so you can house them appropriately. Then, before buying any more, do your research.
As for lighting for plants - fluorescent bulbs are best; some say they are for aquariums and/or for plants, but they tend to be more expensive than necessary. Look for bulbs that say "daylight," or "sunlight" or "full spectrum" on them. They tend to have a bluish cast, sometimes obvious on the unlit bulb, sometimes only visible when lit and in comparison to the yellowish light of incandescent bulbs. By the way, thanks to cfl bulbs, you can use fluorescent bulbs in hoods intended for incandescent bulbs.
Also, make sure your plants and your fish need similar lighting requirements. Some fish need low light, so if you're using live plants, you need low light plants, or if your plants need higher light level than your fish, provide places for them to get out of the light, like shelves, caves, etc.