What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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What species of rainbowfish did you want to keep?
Many rainbowfishes come from hard water with a high pH, whereas emperor tetras come from soft water with a low pH. Some rainbowfish can get quite aggressive and will eat smaller fish including tetras. Emperor tetras are deeper bodied than cardinal tetras but small emperor tetras could get picked on or eaten by some species of rainbowfish.
As a general rule I would not keep Most rainbowfish with tetras. Rhadinocentrus ornatus and Iriatherina werneri come from soft acid water and are smaller and more peaceful and would probably work with emperor tetras.
The following link has information on all the different rainbowfish species. Click on Content in the top right to see the different species. There is also a link at the bottom of the page that will allow you to download the book onto your hard drive. It's about 198MB
A website dedicated to Rainbowfishes of Australia and New Guinea
rainbowfish.angfaqld.org.au