I just learned about cycling but I already have fish. What now?! - Aquarium Advice
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By the sounds of things, you didn't cycle your aquarium properly before adding fish. Do you have a master test kit? (ie API Freshwater master test kit). It is essential, especially for a new aquarium.
What is your level of knowledge, would you say? It would help us to better assist you. What do you already know? Some people don't know to use dechlorinator with tap water, for example.
What make and model of filter and heater do you have with that aquarium by the way? Just curious.
Welcome to the forum! Hope we can help!