What are the tank dimensions (length x width x height)?
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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).
Depending on what the GH of your water is, will determine what fish you should keep.
Tetras, barbs, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm).
Livebearers (guppies, platies, swordtails, mollies) occur in medium hard water with a GH around 200-250ppm.
If you have very hard water (GH above 300ppm) then use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
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What do you feed the fish and how often do you feed them?
How often do you clean the filter and how do you clean it?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
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Try to quarantine all new fish, plants and shrimp for at least 2 (preferably 4) weeks before adding them to the main display tank.