1 - Water changes: the first and the most important rule is to change the entire water of the tank every month – 25% each weak is a reasonable solution. The water changes helps to remove contaminants and waste byproducts from the aquarium and replace them with fresh clean water
2. - Consistent Temperature: fish cannot regulate body temperature so changes to the water temperature are creating a lot of stress for the fish. You'll have to ask for the right temp for your fish and make sure that all your fish can live at the same temp range. Be careful during water changes not to add too cold or too warm water.
3. - Don’t use too many chemicals or products: Chemicals are not very helpful for a healthy tank and this includes pH adjusters and medicines. The only chemicals that go into my tanks are tap water, water conditioner to remove chlorine, charcoal in the filters and aquarium salt (if needed). If you need to adjust the pH of the water, there are natural things you can use such as rocks or substrate to raise the pH or wood or plants to lower it. This way you'll stabilize the pH without creating too much ups and downs.
4. - Use a quarantine tank: This is really important. If your tank is healthy and stable the highest risk for a disease is bringing in new fish that you just brought home from the store. Even at the best pet stores fish can get disease so always keep the newly bought fish at a quarantine tank for at least 3 weeks. This goes for other purchases too such as rocks, plants and anything else you introduce to you tank.
5. Compatible fish: It is important to enable the fish to live peacefully if you have one aggressive fish that is attacking another, know that stress leads to disease and attacks can end up in dead fish. Try to buy compatible fish choosing a fish not just for its beauty but for its ability to live with the other fish that you have. Create hiding places for the fish that needs its and if you have no choice try trading the aggressive fish for a more suitable one.