Hi and welcome to the forum
Check your aquarium water and your tap water for ammonia, nitrite, nitrate and pH.
Depending on where you live, you might have nitrates in the tap water.
If your tank is new and the filter is still cycling (developing the beneficial bacteria used to convert ammonia into nitrite, and nitrite into nitrate), you might have a nitrite reading in the water and this will show up as nitrate. Nitrate test kits read nitrite as nitrate and can give you a false reading if there is nitrite in the water.
Small water changes don't do anything. Bigger water changes dilute things much more effectively. I usually do a 75% water change every day until the levels are 0ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.