Once the undergravel filter is operating properly, you're ready for the final steps in aquarium setup.
First, the power filter can be started. The filter box or canister must be at least half full of water in order to prime the filter. The body of some filters is marked where the water level should be.
Plug the filter in. The filter will fill with water to its normal operating level while any air in the system is expelled. After this has happened, the filter should run smoothly.
The heater can be plugged in now. When the water temperature of the tank is already at or very close to where it should be, setting the thermostat is easy. When the thermometer says that the water temperature is in the proper range, adjust the heater control until the heater's indicator light comes on.
Then, turn the knob the other way just enough for the light to go out. Over the next day or so, check two or three times to see if the heater is maintaining the temperature as it should be. It may take a couple of days to get the heating unit set at the correct level.
Add enough water conditioner for the volume of water in the tank. The hood can be closed and the light turned on. Clean the glass and then stand back and admire your work.
Let the tank run for at least a day and keep an eye on things. Check the temperature and note that the filters are operating properly. Don't be surprised if the water becomes cloudy on the second day. This is most likely the result of bacteria responding to light and warmth. It will clear up in a few days.
Only when you are sure that the tank is running properly and will be a healthy aquatic home should you actually go shopping for your first fish. While you wait, spend some time learning as much about aquarium fish as you can.
Find a step-by-step overview to setting up your aquarium in our final section.