Hi and welcome to the forum
How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the water before adding it to the tank?
What sort of filter is on/ in the tank?
How often do you clean the filter and how do you clean it?
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As a general rule you want to change about 75% of the water each week. Gravel clean the substrate when you do a water change.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Power filters and sponge filters should be cleaned at least once a month and preferably every 2 weeks by washing the filter media in a bucket of tank water. The filter case and impellor (in power filters) can be washed under tap water.
If you check the nitrates they can indicate lack of water changes. You want nitrates to remain below 20ppm and if they are going well above this between water changes, then do more frequent water changes or reduce the food going into the tank.
