Hi and welcome to the forum
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.
Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
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You want to change the light colours so it looks white. You can have red, green, blue, yellow and white light but the colour should appear white. Fish don't like blue or purple light on its own.
If you have live plants, they also need red, blue, green and white light.
For plants, the red & blue light should be even. Green can also be even and white can be added to make it look white.
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Some dirt substrates can continue to release ammonia for months so monitor the ammonia levels and if you have ammonia readings for months, it is probably form the substrate.