Hi and welcome to the forum
I agree with Byron on not keeping dwarf gouramis and rainbowfish. Dwarf gouramis are regularly infected with the Iridovirus and or Tuberculosis (TB). Rainbowfish are prone to TB and keeping the 2 species together is asking for trouble.
Rainbowfish and Corydoras need to be kept in groups of at least 6 or more of their own kind.
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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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How long has the tank been set up for?
Has the filter cycled?
Do you have any plants in the tank?
What are you feeding the rainbowfish?