cold wter orb aquarium

Question
do i have to leave pump filter on all of the time i have only 2 fish  its a large orb tank??

Answer
In order to keep your fish healthy, they need to have an established filter with beneficial bacteria in it.  In order to accomplish this, the filter must be left on at all time.  After a few hours, the bacteria begins to die off and you would have to start the process all over again.  In a fish tank, when a fish uses the bathroom, the waste gets converted to ammonia, which is toxic to fish.  There is a way to fix this, however.  A good bacteria in your tank will "eat" the ammonia and turn it into nitrite.  This too is harmful to fish.  Another bacteria will "eat" the nitrite and make it into nitrate, which you will get out by doing weekly water changes.  In order to get the bacteria built up to the levels that your fish tank needs, you need to keep the filter on as the bacteria attach to the filter and from there reduce the ammonia into nitrite and again into nitrate.  

What kind of fish do you have?
How many gallons does the tank hold?
How long have you had your aquarium?

If you have only had the aquarium for a little while, I would suggest buying liquid reagent test kits to test for ammonia, nitrite, and nitrate.  When the ammonia gets above .25 you need to do a water change.  When the nitrite gets above .1 you also change your water. When the nitrate is above 10 do another water change.  It usually take about 6 weeks for the bacteria to get the ammonia and nitrite under control.  In the meantime, do not add any more fish. If these fish are goldfish, they need at least a 20 gallon tank.

An orb tank will only hold a small school of small tetras or rasboras.  You could also have a SINGLE gourami or betta in there.  If you have anything other than goldfish or white clouds, you really need a heater.  

I know this is a lot of information, but I want you to be successful with your fish and I want your fish to be happy and healthy.  Feel free to follow up with any other questions you might have.