You know what's useless? Common knowledge. The sort of knowledge that makes claims, but never supports them. Aquarium filters can be loud, even if you buy a good one, and some people just want to be able to turn the filter off at night.

Now the common response to the 'My filter is too loud and my boyfriend is going to murder all the fish if they keep him awake for one more night,' dilemma is to say 'Get him some ear plugs,' or 'Buy a better filter'. Both of those options are viable, and if your filter is unsuited for your tank, then sure, you might be better off buying a new filter.

Then again, you might like your filter. You might have spent good money on it. You might not want to buy an endless stream of filters hoping that one will be quiet enough not to disturb your night's sleep. In this case, your desire to turn the filter off at night may very well overwhelm your desire to keep your fish 100% healthy.

The problem with turning off the filter isn't that the ammonia and wastes will build up to toxic levels overnight, it is rather that (according to some) if you turn your filter off overnight you will kill the bacteria in your filter. Your tank will cycle all over again, ammonia will build up and your fish will die. DIE! (The bacteria in the filter are responsible for converting ammonia into less toxic compounds, but let's face it, if they threaten to die after an 8 hour fast, I don't know how responsible they can really be considered to be.)

In my experience, bacteria are normally more robust than to die if they are not fed for 8 hours. This is assuming you are using an internal filter or a filter that remains saturated in the tank water all night, so they don't dry out and become inert.

In the case of filters which stay wet, bacteria will still be floating in a tank full of ammonia, its just that water won't be drawn past them. Thus, I posit that it should be a okay to turn the filter off overnight. However, unlike other online sources (I'm looking at you, Yahoo Answers,) who simply make statements without evidence, I plan to show my working on this front. It may be that I am wrong, in which case we will all know for sure that you must run your filter all night.

I will be cycling my tank, turning it off from 11pm to 8am, and then leaving the filter to run for the rest of the day. To make things a little more difficult, I am going to be starting a cycle from scratch, meaning my bacteria will not have built up at all when this experiment begins. We'll know if the bacteria die every night, because my tank will never cycle properly if this is the case.

Water will be tested at 8am , before the filter is turned on, and at 10.59 pm, before the filter goes off. If, as people claim, ammonia convering bacteria, (reputed to be http://microbewiki.kenyon.edu/index.php/Nitrosococcus) Ammonia will simply build up to toxic levels, and because I keep interrupting and killing my bacteria every evening, it will not come down to 0, as it should.

Stay tuned, this experiment will run for an entire month.

(Oh, and fear not, this is a fishless cycle, so no innocent fishies will be harmed in this experiment.)